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Antibiotic resistance
The antibiotic resistance genes themselves are many and varied, ranging
from plasmid-encoded betalactamases which destroy penicillins to
membrane proteins which reduce the intracellular accumulation of
tetracycline.
Although plasmid-borne resistance to some drugs such as nalidixic acid
and rifampicin does not seem to occur
Colicins and bacteriocins
One group of such proteins, produced by strains of E. coli, are capable of
killing other E. coli strains, and are hence referred to as colicins, and the
strains that produce them are colicinogenic (ColE1).
Virulence determinants
The previous chapter discussed how bacteriophages can carry genes that
code for
toxins and that the presence of the phage is necessary for pathogenicity
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Plasmids in plant-associated bacteria
A different type of pathogenicity is seen with the plant pathogen Agrobacterium
tumefaciens, which causes a tumour -like growth known as a crown gall in some plants
Again, it is only strains that carry a particular type of plasmid (known as a Ti plasmid, for
Tumour Inducing) that are pathogenic; in this case however, pathogenicity is associated
with the transfer of a specific part of the plasmid DNA itself into the plant cells.
Metabolic activities
Plasmids are capable of expanding the host cell’s range of metabolic activities in a variety
of other ways. For example, a plasmid that carries genes for the fermentation of lactose, if
introduced into a lactose non-fermenting strain, will convert it to one that is able to utilize
lactose.
Commonly the potentially pathogenic Salmonella genus is differentiated from the (usually)
non-pathogenic E. coli species primarily because of the inability of Salmonella to ferment
lactose. In some cases, the detection of serious epidemics of Salmonella infections has been
delayed because the causative agent had acquired a lactose-fermenting plasmid.
Biodegradation and bioremediation
Another type of plasmid-mediated metabolic activity is the ability to degrade potentially
toxic chemicals. One such plasmid, pWWO, obtained from Pseudomonas putida, encodes a
series of enzymes that convert the cyclic hydrocarbons toluene and xylene to benzoate
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Degradation of cyclic hydrocarbons
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Molecular properties of plasmids
*In many cases , they are quite small molecules, just a few kilo
bases in length, but in some organisms notably members of
the genus Pseudomonas, plasmids up to several hundred kilo
bases are common
*It is convenient to regard plasmids from E. coli as consisting
of two types
*first
group, of which ColE1 is the prototype, are relatively
small (usually less than 10 kb), and are present in multiple
copies within the cell
*The second group of plasmids, exemplified by the F plasmid,
are larger (typically greater than 30 kb; F itself is about 100
kb) and are present in only one or two copies per cell
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Partitioning of plasmids at cell division
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Plasmid replication and control
Unidirectional Replication
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Plasmid replication and control
Bidirectional Replication
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Plasmid replication and control
Rolling circle Replication
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Replication of
ColE1
Genetic map of the plasmid ColE1. colE1, imm: genes for production of, and
immunity to, colicin E1; mob codes for a nuclease required for mobilization;
rom codes for a protein required for effective control of copy number; ori T:
origin of conjugal transfer; ori V: origin of replication
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Structure and control of the origin of replication of the ColE1 plasmid.
RNA II, after cleavage by RNaseH, acts as a primer for DNA synthesis.
RNA I binds to RNA II and prevent RNase cleavage and hence prevents
initiation of replication
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Replication of R100
Genetic map of the conjugative E.
coli plasmid R100. Resistance
genes: cat, chloramphenicol
(chloramphenicol
acetyltransferase);mer, mercuric
ions; str, streptomycin; sul,
sulphonamides; tet, tetracycline.
Other sites: oriT, origin of
conjugative transfer; repA/ori V,
replication functions and origin
of replication. IS1, IS10 are
insertion sequences, Tn10 is a
transposon
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*Replication control of the plasmid R100. The RepA protein
is needed for initiation of replication. Transcription of repA
is repressed by CopB and translation of the repA mRNA is
inhibited by the antisense copA RNA
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Coupling model for the control
of iteron-containing plasmids
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