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Bacterial conjugation
7
Table of Contents
In this page
This section and subsequent sections describe the discovery of gene transfer in bacteria and explain
several types of gene transfer and their use in bacterial genetics. First, we shall consider conjugation,
which requires cell-to-cell contact. Conjugation was the first extensively studied method of gene
transfer.
Discovery of conjugation
Requirement for physical
contact
Discovery of the fertility factor
Discovery of conjugation
Do bacteria possess any processes similar to sexual reproduction and
(F)
Hfr strains
Determining linkage from
interrupted-mating experiments
recombination? The question was answered in 1946 by the elegantly simple
experimental work of Joshua Lederberg and Edward Tatum, who studied two
strains of Escherichia coli with different nutritional requirements. Strain A would
grow on a minimal medium only if the medium were supplemented with
methionine and biotin; strain B would grow on a minimal medium only if it were
supplemented with threonine, leucine, and thiamine. Thus, we can designate
Figure 7-2
Demonstration
by Lederberg
and Tatum of
genetic
recombination
(more...)
strain A as met − bio − thr+ leu + thi+ and strain B as met + bio + thr− leu − thi− .
Chromosome circularity and
integration of F
R factors
Mechanics of transfer
E. coli conjugation cycle
Figure 7-2a displays in simplified form the concept of their experiment. Here,
Recombination between marker
strains A and B are mixed together, and some of the progeny are now wild type,
genes after transfer
having regained the ability to grow without added nutrients. Figure 7-2b
Gradient of transfer
illustrates their experiment in more detail.
Lederberg and Tatum plated bacteria into dishes containing only unsupplemented
minimal medium. Some of the dishes were plated only with strain A bacteria,
Determining gene order from
gradient of transfer
Higher-resolution mapping by
some only with strain B bacteria, and some with a mixture of strain A and strain B bacteria that had
recombinant frequency in
been incubated together for several hours in a liquid medium containing all the supplements. No
bacterial crosses
colonies arose on plates containing either strain A or strain B alone, showing that back mutations
cannot restore prototrophy, the ability to grow on unsupplemented minimal medium. However, the
plates that received the mixture of the two strains produced growing colonies at a frequency of 1 in
every 10,000,000 cells plated (in scientific notation, 1 × 10−7). This observation suggested that some
form of recombination of genes had taken place between the genomes of the two strains to produce
prototrophs.
Requirement for physical contact
It could be suggested that the cells of the two strains do not really exchange
genes but instead leak substances that the other cells can absorb and use for
growing. This possibility of “cross feeding” was ruled out by Bernard Davis. He
constructed a U-tube in which the two arms were separated by a fine filter. The
pores of the filter were too small to allow bacteria to pass through but large
Figure 7-3
Experiment
demonstrating
that physical
contact
between
(more...)
enough to allow easy passage of the fluid medium and any dissolved substances
(Figure 7-3 ). Strain A was put in one arm; strain B in the other. After the strains
had been incubated for a while, Davis tested the content of each arm to see if
cells had become able to grow on a minimal medium, and none were found. In
other words, physical contact between the two strains was needed for wild-type
cells to form. It looked as though some kind of gene transfer had taken place,
and genetic recombinants were indeed produced.
Discovery of the fertility factor (F)
In 1953, William Hayes determined that genetic transfer occurred in one direction in the above types of
crosses. Therefore, the transfer of genetic material in E. coli is not reciprocal. One cell acts as donor,
and the other cell acts as the recipient. This kind of unidirectional transfer of genes was originally
compared to a sexual difference, with the donor being termed “male” and the recipient “female.”
However, this type of gene transfer is not true sexual reproduction. In bacterial gene transfer, one
organism receives genetic information from a donor; the recipient is changed by that information. In
sexual reproduction, two organisms donate equally (or nearly so) to the formation of a new organism,
but only in exceptional cases is either of the donors changed.
MESSAGE
The transfer of genetic material in E. coli is not reciprocal. One cell acts as the donor, and
the other cell acts as the recipient.
Loss and regain of ability to transfer.
By accident, Hayes discovered a variant of his original donor strain that would not produce recombinants
on crossing with the recipient strain. Apparently, the donor-type strains had lost the ability to transfer
genetic material and had changed into recipient-type strains. In his analysis of this “sterile” donor
variant, Hayes realized that the fertility (ability to donate) of E. coli could be lost and regained rather
easily. Hayes suggested that donor ability is itself a hereditary state imposed by a fertility factor (F).
Strains that carry F can donate, and are designated F + . Strains that lack F cannot donate and are
recipients. These strains are designated F − .
Transfer of F during conjugation
Recombinant genotypes for marker genes are relatively rare in bacterial crosses,
Hayes noted, but the F factor apparently was transmitted effectively during
Figure 7-4
Bacteria can
transfer
plasmids
physical contact, or conjugation. A kind of “infectious transfer” of the F factor
seemed to be taking place. We now know much more about the process of
conjugation and about F, which is an example of a plasmid that can replicate in
the cytoplasm independently of the host chromosome. Figures 7-4 and 7-5 show
(circles of
how bacteria can transfer plasmids such as F. The F plasmid directs the synthesis
DNA), through
of pili, projections that initiate contact with a recipient (Figure 7-4 ) and draw it
(more...)
closer, allowing the F DNA to pass through a pore into the recipient cell. One
strand of the double-stranded F DNA is transferred and then DNA replication
restores the complementary strand in both the donor and the recipient. This
replication results in a copy of F remaining in the donor and another appearing in
Figure 7-5
the recipient, as shown in Figure 7-5 .
(a) During
conjugation,
the pilus pulls
two bacteria
(more...)
Hfr strains
An important breakthrough came when Luca Cavalli-Sforza discovered a
derivative of an F + strain. On crossing with F − strains this new strain produced
1000 times as many recombinants for genetic markers as did a normal F + strain.
Cavalli-Sforza designated this derivative an Hfr strain to indicate a high frequency
of recombination. In Hfr × F − crosses, virtually none of the F − parents were
converted into F + or into Hfr. This result is in contrast with F + × F − crosses,
where infectious transfer of F results in a large proportion of the F − parents being
converted into F + . Figure 7-6 portrays this concept. It became apparent that an
Hfr strain results from the integration of the F factor into the chromosome, as
Figure 7-6
pictured in Figure 7-6a .
The transfer of
E. coli
chromosomal
markers
Now, during conjugation between an Hfr cell and a F − cell a part of the
mediated
before the entire chromosome is transferred. The chromosomal fragment can then
(more...)
recombine with the recipient chromosome. Clearly, the low level of chromosomal
chromosome is transferred with F. Random breakage interrupts the transfer
marker transfer observed by Lederberg and Tatum (see Figure 7-2 ) in an F + × F −
cross can be explained by the presence of rare Hfr cells in the population. When these cells are isolated
and purified, as first done by Cavalli, they now transfer chromosomal markers at a high frequency,
because every cell is an Hfr.
Determining linkage from interrupted-mating
experiments
The exact nature of Hfr strains became clearer in 1957, when Elie Wollman and
François Jacob investigated the pattern of transmission of Hfr genes to F − cells
during a cross. They crossed Hfr str s a+ b + c+ d + with F − str r a− b − c− d − . At
specific time intervals after mixing, they removed samples. Each sample was put
in a kitchen blender for a few seconds to disrupt the mating cell pairs and then
was plated onto a medium containing streptomycin to kill the Hfr donor cells. This
Figure 7-7
Interruptedmating
conjugation
experiments
procedure is called interrupted mating. The str r cells then were tested for the
presence of marker alleles from the donor. Those str r cells bearing donor marker
alleles must have taken part in conjugation; such cells are called exconjugants.
Figure 7-7a shows a plot of the results; azi r, ton r, lac + , and gal + correspond to
with E.
the a+ , b + , c+ , and d + mentioned in our generalized description of the
(more...)
experiment. Figure 7-7b portrays the transfer of markers.
The most striking thing about these results is that each donor allele first appeared
in the F − recipients at a specific time after mating began. Furthermore, the donor alleles appeared in a
specific sequence. Finally, the maximal yield of cells containing a specific donor allele was smaller for
the donor markers that entered later. Putting all these observations together, Wollman and Jacob
concluded that gene transfer occurs from a fixed point on the donor chromosome, termed the origin
(O), and continues in a linear fashion.
MESSAGE
The Hfr chromosome, originally circular, unwinds and is transferred to the F − cell in a linear
fashion. The unwinding and transfer begin from a specific point at one end of the integrated
F, called the origin or O. The farther a gene is from O, the later it is transferred to the F − ;
the transfer process most likely will stop before the farthermost genes are transferred.
Wollman and Jacob realized that it would be easy to construct linkage maps from
the interrupted-mating results, using as a measure of “distance” the times at
Figure 7-8
which the donor alleles first appear after mating. The units of distance in this
Chromosome
case are minutes. Thus, if b + begins to enter the F − cell 10 minutes after a+
map of Figure
begins to enter, then a+ and b + are 10 units apart (Figure 7-8 ). Like the maps
7-7. A linkage
map can be
based on crossover frequencies, these linkage maps are purely genetic
constructions; at the time, they had no known physical basis.
(more...)
Chromosome circularity and integration of F
When Wollman and Jacob allowed Hfr × F − crosses to continue for as long as 2 hours before blending,
they found that a few of the exconjugants were converted into Hfr. In other words, an important part of
F (the terminal part now known to confer “maleness,” or donor ability), was eventually being
transmitted but at a very low efficiency, and it apparently was transmitted as the last element of the
linear chromosome. We now have the following map, in which the arrow indicates the process of
transfer, beginning with O:
However, when several different Hfr linkage maps were derived by interrupted-mating and time-of-entry
studies using different, separately derived Hfr strains, the maps differed from strain to strain.
At first glance, there seems to be a random reshuffling of genes. However, a pattern does exist; the
genes are not thrown together at random in each strain. For example, note that in every case the his
gene has gal on one side and gly on the other. Similar statements can be made about each gene,
except when it appears at one end or the other of the linkage map. The order in which the genes are
transferred is not constant. In two Hfr strains, for example, the his gene is transferred before the gly
gene (his is closer to O), but, in three strains, the gly gene is transferred before the his gene.
How can we account for these unusual results? Allan Campbell proposed a
startling hypothesis: suppose that, in an F + male, F is a small cytoplasmic
element (and therefore easily transferred to an F − cell on conjugation). If the
Figure 7-9
Circularity of
the E. coli
chromosome.
(a) Through
(more...)
chromosome of the F + male were a ring, any of the linear Hfr chromosomes
could be generated simply by inserting F into the ring at the appropriate place
and in the appropriate orientation (Figure 7-9 ).
Several conclusions—later confirmed—follow from this hypothesis.
1.
The orientation in which F is inserted would determine the polarity of the Hfr chromosome, as
indicated in Figure 7-9a .
2.
At one end of the integrated F factor would be the origin, where transfer of the Hfr
chromosome begins; the terminus at the other end of F would not be transferred unless all the
chromosome had been transferred. Because the chromosome often breaks before all of it is
transferred and because the F terminus is what confers maleness, then only a small fraction of
the recipient cells would be converted into male cells.
How, then, might F integration be explained? Wollman and Jacob
suggested that some kind of crossover event between F and the F +
chromosome might generate the Hfr chromosome. Campbell then came
Figure 7-10
Insertion of the
F factor into the
up with a brilliant extension of that idea. He proposed that, if F, like the
chromosome, were circular, then a crossover between the two rings would
produce a single larger ring with F inserted (Figure 7-10 ).
E. coli
chromosome
Now suppose that F consists of three different regions, as shown in
(more...)
Figure 7-10 . If the bacterial chromosome had several homologous regions
that could match up with the pairing region of F, then different Hfr
chromosomes could be easily generated by crossovers at these different sites.
Chromosomal and F circularity were wildly implausible concepts initially, inferred solely from the
genetic data; confirmation of their physical reality came only a number of years later. The
direct-crossover model of integration also was subsequently confirmed.
The fertility factor thus exists in two states: (1) the plasmid state, as a free cytoplasmic element
F that is easily transferred to F − recipients, and (2) the integrated state, as a contiguous part of
a circular chromosome that is transmitted only very late in conjugation. The word episome
(literally, “additional body”) was coined for a genetic particle having such a pair of states. A cell
containing F in the first state is called an F + cell, a cell containing F in the second state is an
Hfr cell, and a cell lacking F is an F − cell. Today the term plasmid is used to refer to any selfreplicating circular element in the cytoplasm and “episome” is rarely used.
R factors
A frightening ability of pathogenic bacteria was discovered in Japanese hospitals in the 1950s. Bacterial
dysentery is caused by bacteria of the genus Shigella. This bacterium initially proved sensitive to a wide
array of antibiotics that were used to control the disease. In the Japanese hospitals, however, Shigella
isolated from patients with dysentery proved to be simultaneously resistant to many of these drugs,
including penicillin, tetracycline, sulfanilamide, streptomycin, and chloramphenicol. This multiple-drugresistance phenotype was inherited as a single genetic package, and it could be transmitted in an
infectious manner—not only to other sensitive Shigella strains, but also to other related species of
bacteria. This talent is an extraordinarily useful one for the pathogenic bacterium, and its implications
for medical science were terrifying. From the point of view of the geneticist, however, the situation is
very interesting. The vector carrying these resistances from one cell to another proved to be a selfreplicating element similar to the F factor. These R factors (for “resistance”) are transferred rapidly on
cell conjugation, much like the F particle in E. coli.
In fact, these R factors proved to be just the first of many similar F-like factors
to be discovered. These elements, which exist in the plasmid state in the
cytoplasm, have been found to carry many different kinds of genes in bacteria.
Table 7-2 shows some of the characteristics that can be borne by plasmids.
Table 7-2
Genetic
Determinants
Borne by
Plasmids
Mechanics of transfer
Does an Hfr cell die after donating its chromosome to an F − cell? The answer is no (unless the culture
is treated with streptomycin). The Hfr chromosome replicates while it is transferring a single strand to
the F − cell; this replication ensures a complete chromosome for the donor cell after mating. The
transferred strand is replicated in the recipient cell, and donor genes may become incorporated in the
recipient’s chromosome through crossovers, creating a recombinant cell. Otherwise, transferred
fragments of DNA in the recipient are lost in the course of cell division.
We assume that the F − chromosome is also circular, because the recipient F − cell, if it receives the F
factor from an F + cell, is readily converted into an F + cell from which an Hfr cell can be derived.
The picture emerges of a circular Hfr chromosome unwinding a copy of itself, which is then transferred
in a linear fashion into the F − cell. How is the transfer achieved? Electron microscope studies show that
Hfr and F + cells have fibrous structures, F pili, protruding from their cell walls, as shown in Figure 7-4 .
The F pili facilitate cell-to-cell contact, during which DNA is transferred through pores in the F − .
E. coli conjugation cycle
We can now summarize the various aspects of the conjugation cycle in E. coli
(Figure 7-11 ). We shall review the conjugation cycle in regard to the differences
between F − , F + , and Hfr cells, because these differences epitomize the cycle.
Figure 7-11
Summary of
the various
events that
take place in
the (more...)
F − strains do not contain the F factor and cannot transfer DNA by conjugation. They are,
however, recipients of DNA transferred from F + or Hfr cells by conjugation.
F + cells contain the F factor in the cytoplasm and can therefore transfer F in a highly efficient
manner to F − cells during conjugation.
Hfr cells have F integrated into the bacterial chromosome, not in the cytoplasm.
Chromosomal markers are transferred in a strain of F + cells because, in any population of F + cells, a
small fraction of cells (about 1 in 1000) have been converted into Hfr cells by the integration of F into
the bacterial chromosome. Because conjugation experiments are usually carried out by mixing from 107
to 10 8 cells consisting of prospective donors and recipients, the population will contain various different
Hfr cells derived from independent integrations of F into the chromosome at various different sites.
Therefore, when chromosomal markers are transferred by different cells in the population, transfer will
start at different points on the chromosome. This results in an approximately equal transfer of markers
all around the chromosome, although at a low frequency. This type of F + -mediated transfer is what
Lederberg and Tatum observed when they discovered gene transfer in bacteria.
Each of the Hfr cells in an F + population with an integrated F factor can be the source of a new Hfr
strain if it is isolated and used to start a clone.
Hfr strains are derived from a clone of Hfr cells in which a specific integration of F into the bacterial
chromosome has taken place. Therefore, all the cells in any given Hfr strain have F integrated into the
chromosome at exactly the same point.
Hfr populations transfer chromosomal markers to F − cells at a high frequency compared with F +
populations, because only a fraction of cells in an F + population have F integrated into the
chromosome. Further, in any given Hfr strain, the markers are transferred from a fixed point in a
specific order. This also contrasts with F + populations, where the Hfr cells transfer chromosomal
markers in no particular fixed order, given that the F factor integrates into the chromosome at different
points in different F + cells.
In an Hfr × F − cross, the F − is not converted into Hfr or into F + , except in very rare cases, because the
Hfr chromosome nearly always breaks before the F terminus is transferred to the F− cell.
Recombination between marker genes after transfer
Thus far, we have studied only the process of the transfer of genetic information between individuals in
a cross. This transfer is inferred from the existence of recombinants produced from the cross. However,
before a stable recombinant can be produced, the transferred genes must be integrated or incorporated
into the recipient’s genome by an exchange mechanism. We now consider some of the special
properties of this exchange event.
Genetic exchange in prokaryotes does not take place between two whole
genomes (as it does in eukaryotes); rather, it takes place between one complete
genome, derived from F − , called the endogenote, and an incomplete one, derived
from the donor, called the exogenote. What we have in fact is a partial diploid, or
merozygote. Bacterial genetics is merozygous genetics. Figure 7-12a is a diagram
Figure 7-12
Crossover
between
exogenote and
endogenote in a
merozygote
(more...)
of a merozygote.
A single crossover would not be very useful in generating viable recombinants,
because the ring is broken to produce a strange, partly diploid linear
chromosome (Figure 7-12b). To keep the ring intact, there must be an even
number of crossovers (Figure 7-12c ). The fragment produced in such a crossover
is only a partial genome, which is generally lost in subsequent cell growth. Hence,
both reciprocal products of recombination do not survive—only one does. A
further unique property of bacterial exchange, then, is that we must forget about
reciprocal exchange products in most cases.
MESSAGE
In the genetics of bacteria, we generally are concerned with double crossovers and we do not
expect reciprocal recombinants.
Gradient of transfer
Only partial diploids exist in the merozygote. Some genes don’t even get into the act. To better
understand this fact, let us look again at the consequences of gene transfer. Usually, only a fragment of
the donor chromosome appears in the recipient, owing to spontaneous breakage of the mating pairs; so
the entire chromosome is rarely transferred. The spontaneous breakage can occur at any time after
transfer begins, which creates a natural gradient of transfer and makes it less and less likely that a
recipient cell will receive later and later genetic markers. (“Later” here refers to markers that are
increasingly farther from the origin and hence are donated later in the order of markers transferred.)
For example, in a cross of Hfr-donating markers in the order met, arg, leu, we would expect a
distribution of fragments such as the one represented here:
Note that many more fragments contain the met locus than the arg locus and that the leu locus is
present on only one fragment. It is easy to see that the closer the marker is to the origin, the greater
the chance it will be transferred in conjugation.
The concept of the gradient of transfer is the same as the one described earlier for interrupted matings,
except that here we are allowing the natural disruption of mating pairs to occur instead of interrupting
the pairs mechanically.
Determining gene order from gradient of transfer
We can use the natural gradient of transfer to establish the order of genetic markers, provided we
select for an early marker that enters before the markers that we are ordering. Let’s see how this
works. Suppose that we use an Hfr strain that donates markers in the order met, arg, aro, his. In a
cross of an Hfr that is met + arg + aro + his + str s with an F − that is met − arg − aro − his − str r,
recombinants are selected that can grow on a minimal medium without methionine but with arginine,
aromatic amino acids, and histidine and in the presence of streptomycin. Here we are selecting for
recombinants in the F − strain that are met + in a cross in which the met locus is transferred as the
earliest marker. We can then score for inheritance of the other markers present in the Hfr by testing on
supplemental minimal medium lacking, in turn, one of the required nutrients.
A typical result would be:
Note how the frequency of inheritance corresponds to the order of transfer. This correspondence is due
to the fact that the frequency of inheritance is indicative of the frequency of transfer. For this method
to work, it is crucial that it be applied only to genetic markers that enter after the selected marker—in
this case, after met.
Higher-resolution mapping by recombinant frequency
in bacterial crosses
Although interrupted-mating experiments and the natural gradient of transfer can give us a rough set of
gene locations over the entire map, other methods are needed to obtain a higher resolution between
marker loci that are close together. Here we consider one approach to the problem: using the frequency
of recombinants to measure linkage.
Previous attempts to measure linkage in conjugational crosses were hindered by the failure to
understand that only fragments of the chromosome are transferred and that the gradient of transfer
produces a bias toward the inheritance of early markers. To measure linkage and to attach any meaning
to a calculated map distance, it is necessary to produce a situation in which every marker has an equal
chance at being transferred so that the recombinant frequencies are dependent only on the distance
between the relevant genes.
Suppose that we consider three markers: met, arg, and leu. If the order is met, arg, leu and if met is
transferred first and leu last, then we want to set up the situation diagrammed here to calculate map
distances separating these markers:
Here, we have to arrange to select the last marker to enter, which in this case is leu. Why? Because, if
we select for the last marker, then we know that every cell that received fragments containing the last
marker also received the earlier markers—namely, arg and met—on the same fragments. We can then
proceed to calculate map distance in the classic manner. Rather than using map units, we simply refer
to the percentage of crossovers in the respective interval on the map. In practice, this is done by
calculating, among the total recovered recombinants, the percentage of recombinants produced by
crossovers between two markers. Let’s look at an example.
Sample cross
In the cross of the Hfr strain just described (met + arg + leu + str s) with an F −
that is met − arg − leu − str r, we would select leu + recombinants and then
examine them for the arg and met markers. In this case, the arg and met
Figure 7-13
Incorporation of
a late
marker into the
F − (more...)
markers are called the unselected markers. Figure 7-13 depicts the types of
crossover events expected. Note how two crossover events are required to
incorporate part of the incoming fragment into the F − chromosome. One
crossover must be on each side of the selected (leu) marker. Thus, in
Figure 7-13 , one crossover must be on the left side of the leu marker and the
second must be on the right side. Suppose that the map distance between each
marker is 5 percent recombination. In 5 percent of the total leu + recombinants, the second crossover
occurs between leu and arg (Figure 7-13a ); in another 5 percent of the cases, the second crossover
occurs between leu and met (Figure 7-13b). We would then expect 90 percent of the selected leu +
recombinants to be arg + met + , because the second crossover occurs outside the leu–arg–met interval
(Figure 7-13c ) in 90 percent of the cases. We would also expect 5 percent of the leu + recombinants to
be arg − met − , resulting from a crossover between leu and arg, and 5 percent of the leu + recombinants
to be arg + met − , resulting from a crossover between arg and met. In reality, then, we are simply
determining the percentage of the time that the second crossover occurs in each of the three possible
intervals.
In a cross such as the one just described, one class of potential recombinants requires an additional two
crossover events (Figure 7-13d). In this case, the leu + arg − met + recombinants would require four
crossovers instead of two. These recombinants are rarely recovered, because their frequency is sharply
reduced compared with the other classes of recombinants.
Infectious marker-gene transfer by episomes
Edward Adelberg’s work led to the discovery of gene transfer at high frequency by episomes. When he
began his recombination experiments in 1959, the particular Hfr strain that he used kept producing F +
cells, so the recombination frequencies were not very large. Adelberg called this particular fertility factor
F′ to distinguish it from the normal F, for the following reasons:
1.
The F′-bearing F + strain reverted back to an Hfr strain much more frequently than do
typical F + strains.
2.
F′ always integrated at the same place to give back the original Hfr chromosome.
(Remember that randomly selected Hfr derivatives from F + males have origins at many different
positions.)
How could these properties of F′ be explained? The answer came from the recovery of an F′ from an Hfr
strain in which the lac + locus was near the end of the Hfr chromosome (it was transferred very late).
Using this Hfr lac + strain, François Jacob and Adelberg found an F + derivative that transferred lac + to
F − lac − recipients at a very high frequency. Furthermore, the recipients that behaved like F + lac +
occasionally produced F − lac − daughter cells, at a frequency of 1 × 10 −3. Thus, the genotype of these
recipients appeared to be F lac + /lac − .
Now we have the clue: F′ is a cytoplasmic element that carries a part of the
bacterial chromosome. In fact, it is nothing more than F with a piece of the host
chromosome incorporated. Its origin and reintegration can be visualized as shown
in Figure 7-14 . This F′ is known as F′ lac, because the piece of host chromosome
that it picked up has the lac gene on it. F′ factors have been found carrying
many different chromosomal genes and have been named accordingly. For
Figure 7-14
example, F′ factors carrying gal or trp are called F′ gal and F′ trp, respectively.
Origin and
reintegration of
the F′ factor,
(more...)
Because F lac + /lac − cells are Lac+ in phenotype, we know that lac + is dominant
over lac − . As we shall see in Chapter 11, the dominant– recessive relation
between alleles can be a very useful bit of information in interpreting gene
function.
Partial diploidy for specific segments of the genome can be made with an array of F′ derivatives from
Hfr strains. The F′ cells can be selected by looking for the infectious transfer of normally late genes in a
specific Hfr strain. Some F′ strains can carry very large parts (up to one-quarter) of the bacterial
chromosome; if appropriate markers are used, the merozygotes generated can be used for
recombination studies.
MESSAGE
During conjugation between an Hfr donor and an F − recipient, the genes of the donor are
transmitted linearly to the F − cell, through the bacterial chromosome, with the inserted
fertility factor transferring last.
In the course of conjugation between an F + donor carrying an F′ plasmid and an F −
recipient, a specific part of the donor genome may be transmitted infectiously to the F − cell,
through the plasmid. The transmitted part was originally adjacent to the F locus in an Hfr
strain from which the F + was derived.
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Sample cross
Infectious marker-gene transfer
by episomes