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GENETIC EXCHANGE
UTILIZING MICROBIAL
DONORS OR VECTORS
MM 28-45
Table of Contents
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Introduction
Genetic Exchange Between Viruses
Genetic Exchange Between Bacteria
Summary
Introduction
Genetic exchange is one mechanism by which new genotypes of species are formed
(mutation is the other mechanism). In the microbial world this genetic exchange may
occur via either an asexual or a sexual process whereas in higher plants and animals it
is usually a sexual process but may also rarely be the result of a viral infection.
Whatever the mechanism of genetic exchange, the final result is an organism (or cell)
with an altered genotype. The newly acquired genes may be either beneficial or
harmful to the organism (e.g., a bacterium may gain antibiotic resistance or an animal
may develop a malignancy). Since genetic exchange continues to play a major role in
determining how medicine is practiced, it is important to understand how genetic
exchange occurs utilizing microbial donors and/or vectors.
Genetic Exchange Between Viruses
Genetic exchange between viruses can only occur when two viruses simultaneously
infect the same cell. Then genetic exchange can occur by:
Recombination
This is the breakage and reunion of homologous regions in the nucleic acid molecules
from two viruses.
This process occurs within almost all groups of DNA-containing viruses and within the
Retroviridae at the DNA provirus stage. The genomes of the Picornaviridae are the only
RNAs known to undergo crossovers among animal viruses.
Reassortment
This is an exchange of nucleic acid segments between viruses with segmented
genomes. This includes only certain RNA-containing viruses (Orthomyxoviridae,
Arenaviridae, Reoviridae and Bunyaviridae). Reassortment is the mechanism of
"genetic shift" whereby influenza viruses rapidly acquire new hemagglutinin and
neuraminidase antigens. This is the initiating factor in many epidemics.
Polyploidy and heteropolyploidy (genotypic mixing)
This is the incorporation of more than one complete genome into the same virus
capsid. If the genomes are from the same virus species, the phenomenon is
polyploidy; if the genomes are from different virus species, the phenomenon is
heteropolyploidy or genotypic mixing. There is no recombination between the
genomes and cells singly infected with the genotypically mixed viruses will yield
progeny identical to both original parents as well as more genotypically mixed viruses.
Genetic Exchange Between Bacteria
Part of the genetic material of a donor cell can be transferred to a recipient cell. After
the transfer, recombination between the donor and recipient DNA may occur followed
by succeeding nuclear and cell division.
The model for recombination is as follows:
Homologous DNA strands pair.
Breakage of two strands occurs.
The broken segments are then reciprocally rejoined.
The crossover point is laterally displaced.
The other strands are broken and partially digested.
Localized repair and reciprocal joining occurs.
The result is recombinant DNA molecules that exhibit heterozygosity at the site of
crossing over.
Transformation
This is the uptake of extracellular DNA by bacteria. This process occurs only in
bacteria that can take-up high molecular weight DNA from the medium. Most bacterial
cells need to be at a particular stage in their growth cycle or under a particular growth
regimen in order to be transformed. Also, certain factors such as DNA binding proteins
of the cell envelope and poly-b-hydroxybutyrate are necessary for transformation.
Transformation is a three step process:
1.
High molecular weight DNA must bind to the cell surface.
2.
The bound DNA is taken up through the cell membrane.
3.
The donor DNA fragment is then integrated into the host
chromosome or replicates autonomously as a plasmid.
Transformation in Gram-positive bacteria
The Gram-positive species that are able to take up exogenous DNA include
Streptococcus pneumoniae, Staphylococcus aureus and Bacillus subtilis. Grampositive strains take up both homologous and heterologous DNA.
Plasmid DNA may also be taken up by competent cells. However, these molecules
must be linearized before cell entry. Plasmids may be restored by recombination
between overlapping plasmid molecules or recombination with the recipient's
chromosome.
Transformation in Gram-negative bacteria
Gram-negative bacteria that can be transformed by exogenous DNA include Neisseria
meningitis, Neisseria gonorrhea, Haemophilus influenzae, and Escherichia coli.
Homologous DNA is taken up at a much higher rate than heterologous DNA.
Transformasomes, membrane bound organelles, sequester DNA and transfer it into the
cell's interior. Degradation of one DNA strand follows and only a single strand
participates in recombination. Uptake specificity may depend upon specific sites in the
donor DNA. Example - Haemophilus influenzae contains 11 base pair sequences that
are essential for recognition and DNA uptake.
Transduction
This genetic transfer occurs in both Gram-positive and Gram-negative bacteria when a
fragment of DNA is carried to the recipient cell by virus (bacteriophage) produced by a
donor cell. Transduction is observed with temperate bacteriophages (those that can
form prophages). A prophage is a bacterial virus that has integrated its DNA into the
DNA of a bacterial cell. This process of integration of viral DNA into bacterial DNA is
lysogenization. Lysogenization that results in a change in the phenotype of the host
cell is called lysogenic conversion. Restricted (specialized) transduction - this occurs
when the transducing phage only carries segments of DNA that are immediately
adjacent to the site of prophage attachment. After phage is introduced into the cell, its
DNA becomes integrated into the bacterial chromosome. When the phage becomes
induced, the phage DNA is excised from the bacterial chromosome, the phage
replicates, and the host cell lyses, releasing mature phage particles. Occasionally, the
excision of the phage DNA is imprecise and the resulting excised piece of phage
contains some of the host bacterial genome. If the host DNA replaces essential phage
genes, the resulting phage will be a defective phage which cannot mature and replicate
unless in the presence of a normal lambda phage. Defective phage are also called
transducing particles.
When transducing particles infect donor cells, the donor DNA integrates into the
bacterial chromosome in the normal fashion. These transduced genes are expressed in
the recipient cell even though they are contained within the prophage DNA.
Generalized transduction is said to occur when the phage has a roughly equal
chance of carrying any segment of the donor's chromosome.
Specialized transduction occurs when a gene or a set of genes has a high
frequency of transduction relative to the majority of genes on the bacterial
chromosome.
Plasmid-Mediated Transfer (conjugation)
Plasmids are small autonomously replicating circular pieces of DNA. These may carry
genes for resistance to drugs or virulence factors. Many of these plasmids also
mediate gene transfer, resulting in bacterial strains with unique drug resistance patters
or novel virulence factors. They contain double-stranded circular DNA.
Plasmid transfer in Gram-negative bacteria occurs only between strains of the same
species or closely-related species.
Many plasmids of Gram-negative bacteria are conjugative. These plasmids carry genes
that mediate their own transfer. These genes code for the production of the sex pilus
and enzymes necessary for conjugation.
Conjugation begins with the extrusion of a sex pilus; the tip of the sex pilus adheres to
the outer membrane of Gram-negative cell walls. Following pilus adherence, the two
cells become bound together at a point of direct envelope-to-envelope contact.
After pair formation, the plasmid will undergo a specific type of replication called
"transfer replication." One parental strand of DNA is broken and passes into the
recipient while the other remains circularized and in the donor. The complementary
strands are synthesized in both donor and recipient cells. Daughter DNA plasmids are
recircularized with ligase immediately after transfer replication is complete.
During conjugation, no cytoplasm or cell material except DNA passes from donor to
recipient. After conjugation, the cells break apart and two plasmid containing cells
result.
Self-transfer in Gram-positive bacteria
Plasmid-mediated conjugation occurs in Bacillus subtilis, Streptococcus lactis,
and Enterococcus faecalis but is not found as commonly in the Gram-positive
bacteria as compared to the Gram-negative bacteria.
Plasmids may integrate into the bacterial chromosome depending upon the
extent of DNA homology between the two. After integration, both plasmid and
chromosome will replicate as a single unit. A plasmid that is capable of
integrating into the chromosome is called an episome.
Some conjugative plasmids are able to integrate into the host chromosome.
After integration, both chromosome and plasmid can be conjugally transferred
to a recipient cell. Plasmids that are able to mobilize chromosomal transfer are
called sex factors or F (fertility) factors. Cells that contain the sex factor F are
designated F+ and those that do not contain the factor are F-. If the F-plasmid is
integrated into the chromosome it is called an Hfr cell.
When Hfr cells are added to an excess of F- cells, all Hfr cells will attach to the
susceptible F- cells and replicative transfer (conjugation) will occur. The
chromosome and the F factor will be transferred to the recipient.
Cell Properties Carried by Plasmids
Drug resistance
Gram-negative bacteria carry plasmids that give resistance to antibiotics such as
neomycin, kanamycin, streptomycin, chloramphenicol, tetracyclines, penicillins and
sulfonamides. Gram-positive Staphylococcus aureus carries plasmids that contain
genes for resistance to penicillin, heavy metals (Hg or Co) and erythromycin. Most
antibiotic resistance in such strains is mediated by enzymes that inactivate the drug by
acetylation or phosphorylation. Chromosomally encoded antibiotic resistance genes
often act by altering the binding site for the antibiotic.
Gram-negative plasmids that contain antibiotic resistance genes are called R factors.
The R factors are composed of the resistance transfer factor (RTF) segment of the
plasmid that contains the genes responsible for intercellular transfer and R
determinant that carries the resistance genes.
Virulence
(1) Toxins - Enterotoxins (Escherichia coli, Vibrio cholerae), exfoliative toxin
(Staphylococcus aureus), dermotoxin of
Bacillus anthracis, the neurotoxin of Clostridium tetani, and the pesticide toxin of
Bacillus thuringiensis.
(2) Adhesins - such as produced by the plasmids of Yersinia enterocolitica, Shigella
flexneri, Escherichia coli strains that
produce dysentery, and Yersinia pestis.
(3) Growth factors - Other plasmid borne virulence factors act to directly aid the
bacteria in competing with mammalian
host cells for growth. For examples, the plasmid Col V of Escherichia coli contains
genes for iron sequestering
compounds. The acquisition of iron is essential for the survival of Escherichia coli
in mammalian infections.
Production of antimicrobial agents
Bacteriocins are a special class of antimicrobics that are active only against other
strains of the same species that produced them. A specific example is the colicins
which are produced by Escherichia coli cells that harbor small nonconjugative
plasmids. These plasmids are called colfactors and contain genes for the colicin it
produces, as well as a gene for a protein that protects the donor cell from the colicin.
Some colicins act by forming ion-permeable channels in the membrane of sensitive
cells. These act to collapse the membrane potential of the colicin sensitive cell.
Metabolic activities
Examples of these genes are those that allow bacteria to utilize unique or unusual
materials for carbon or energy sources. Many of the genes for these metabolic
pathways are on transmissible plasmids.
Examples of metabolic activities determined by plasmids
Organism
Pseudomonas spp
Activity
Degradation of camphor, toluene, octane, salicylic acid
Bacillus
stearothemophilus
-Amylase
Alcaligenes eutrophus
Utilization of H2 as oxidizable energy source
Escherichia coli
Sucrose uptake and metabolism, citrate uptake
Klebsiella spp
Nitrogen fixation
Streptococcus (group N)
Lactose utilization, galactose phosphotransferase system, citrate
metabolism
Rhodospirillum rubrum
Synthesis of photosynthetic pigment
Flavobacterium spp
Nylon degradation
Surface antigens
Genetic Exchange Between Fungi
Classes of fungi based on sexual spore formation
Classes of fungi based on sexual spore formation
• Phycomycetes (Mucor sp. and Rhizopus sp.)
• Ascomycetes (Aspergillus sp. and Penicillium sp.)
• Basideomycetes (mushrooms)
In the phycomyces, sexual reproduction takes place by simple copulation of the tips
of the multinucleate hyphae. The tips consist of terminal swellings and arise as
branches from the mycelial mats. The tips are attracted to one another by sex
hormones or they merely come into contact by chance. After contact, each of the
hyphal tips swells, and a septal wall is formed separating the cytoplasm and nuclei in
the swollen end from the rest of the hypha. The wall between the adjacent tips then
dissolves, and there is mixing of the cytoplasm from the two mating strains, followed
by paring of the nuclei. The new cell (zygote), which is the product of this fusion
enlarges and the walls become thick and pigmented. When nuclear fusion takes place,
a diploid nucleus is formed. After a period of inactivity, the zygote cracks open, a
spormgiophore emerges, and a sporangium develops. The spormgiophores within the
sporangium undergo a nuclear conjugation followed by reduction division to produce
a haploid spore containing genes from both parents.
In the ascomycetes, sexual reproduction systems are quite varied. However, when two
sexual types (+ and -) come together the end result of all these variations is the
formation of the ascus or sac in which the sexual spores (ascospores) are produced.
After nuclear conjugation and reduction division have occurred, the ascus contains
eight haploid nuclei destined to be contained in spores.
In the basideomycetes sexual reproduction occurs only among those species which
form clamp connections as seen below.
Summary
1. Genetic exchange between viruses occurs by recombination, reassortment and
polyploidy.
2. Recombination is the breakage and reunion of homologous regions in the nucleic
acid of two viruses.
3. Reassortment is an exchange of nucleic acid segments between viruses with
segmented chromosomes.
4. Polyploidy is the incorporation of more than one complete genome into the same
virus capsid.
5. Genetic exchange between bacteria occurs by transformation, transduction and
conjugation.
6.
Transformation is the uptake of extracellular DNA by bacteria.
7. Transduction is the transfer of bacterial DNA, by a virus, from one bacterial cell to
another.
8. A prophage is a bacterial virus that has the ability to integrate its DNA into the DNA
of a bacterial cell.
9. Lysogenization is the process of integrating bacteriophage DNA into bacterial
DNA.
10. Lysogenic conversion is a change in the phenotype of a bacterial cell due to
lysogenization.
11. In restricted transduction, only those genes near the prophage attachment site
are transduced.
12. In generalized transduction, all genes have an equal probability of being
transduced.
13. Plasmids are small (relative to the chromosome) autonomously replicating
circular pieces of DNA.
14. A plasmid that is capable of integrating into the chromosome is termed an
episome.
15. Plasmids may carry genes for transfer of DNA (their own DNA as well as
chromosomal DNA) to another bacterial cell, drug resistance, virulence, production of
antimicrobial agents and for metabolic activities.
16.
Bacterial conjugation is plasmid-mediated gene transfer.
17. A plasmid that can mediate gene transfer is termed the F (fertility) plasmid. A
bacterial cell containing the F plasmid is called an F+cell. A bacterial cell not
containing a F plasmid is called the F-cell. A bacterial cell containing a F plasmid
integrated into the bacterial DNA is termed a Hfr (high frequency of recombination)
cell.
18. Bacteriocins are antimicrobics that are active only against other strains of the
same species that produced them.
19. In general, plasmids can carry genes coding for drug resistance, virulence,
antimicrobial agents, and metabolic activities.
20. Genetic exchange between fungi occurs via exchange of whole nuclei. The
method of exchange varies between the Phycomycetes, Ascomycetes and
Basideomycetes but the end result is the same, i.e., formation of a zygote.
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