Extrachromosomal DNA Substances
Plasmids are extrachromosomal DNA substances.
They are replicons that are maintained as discrete, extrachromosomal genetic
elements in bacteria. They are usually much smaller than the bacterial
chromosome, varying from less than 5 to more than several 100 kbp. However,
plasmids as large, as 2 million base pairs can occur in some bacteria. Plasmids
are circular and double-stranded DNA molecules that encode traits that are not
essential for bacterial viability. They are capable of replicating
independently of the bacterial chromosomes. The plasmids can also be present as
integrated with bacterial chromosomes, and plasmids integrated with host
chromosome are known as episomes. Plasmids are present in
both Gram-positive and Gram-negative bacteria.
Plasmids depending on their transmissibility and nature of the
factor can be of the following types:
Plasmids, depending on transmissibility are of two types: (a) transmissible plasmids and (b) nontransmissible plasmids.
Depending on the nature of factors, plasmids are of the following
types: (a) the F factor, (b) the R factor, and (c) the Col factor.
The F factor: The F plasmid, also called F factor, is atransfer
factor that contains the genetic information, essential for controlling mating
process of the bacteria during conjugation.
The F plasmid of Escherichia coli is the prototype for
fertility plasmids in Gram-negative bacteria. Strains of E.coli with an extrachromosomal F plasmid are called F1andfunction as donors,
whereas strains that lack the F plasmid are F− and behave as recipients. The conjugative
functions of the F plasmid are determined by a cluster of at least 25 transfer
(tra) genes. These genes determine (a)
expression of pili, (b) synthesis and
transfer of DNA during mating, (c)
interference with the ability of F1 bacteria to serve as recipients, and (d) other functions.
plasmid in E. coli can occur as an
extrachromosomal genetic element or be integrated into the bacterial
chromo-some. Both the F plasmid and the bacterial chromosome are circular DNA
molecules. Hence, reciprocal recombination between them produces a larger DNA
circle consisting of F-plasmid DNA inserted linearly into the chromosome.
The R factor: Resistance factors, also called R factors,are extrachromosomal
plasmids. They are circular with double-stranded DNA. R factors occur in two
sizes: large plasmids (mol. wt. 60 million) and small plasmids (mol. wt. 10
million). The large plasmids are conjugative “R” factors, which contain extra
DNA to code for the conjugation process. The small plasmids contain only the
“r” genes and are not conjugative. R factor consists of two components: the
resistance transfer factor (RTF) and resistant determinant (r). The RTF is
responsible for conjugational transfer, while each r determinant carries resistance
for one of the several antibiotics.
Colicinogenic (Col) factor: Col factor is a plasmid
thatresembles the F factor in promoting conjugation, leading to self-transfer
and also at times transfer of segments of chromosomes.
The Col factor encodes for production of colicins, which are
antibiotics-like substances that are specifically and selectively lethal to
other enteric bacteria.
They also encode for production of diphthericin and pyocyanin
produced by Corynebacterium diphtheriae
and Pseudomonas pyocyanea,
respectively, which are sub-stances similar to colicins.
Many plasmids control medically important properties of pathogenic
bacteria. These include (a)
resistance to one or several antibiotics, (b)
production of toxins, and (c)
synthesis of cell sur-face structures required for adherence or colonization.
Some plasmids are cryptic and have no recognizable effects on the bacterial
cells that harbor them. Comparing plasmid profiles is a useful method for
assessing possible relatedness of individual clinical isolates of a particular
bacterial species for epidemiological studies.