I. MICROBIAL GENETICS
G. Genetic Recombination in Bacteria
LEARNING OBJECTIVES FOR THIS SECTION
Horizontal Gene Transfer in Bacteria (def)
Horizontal gene transfer (def), also known as lateral gene transfer, is a process in which an organism transfers genetic material to another organism that is not its offspring. The ability of Bacteria and Archaea to adapt to new environments as a part of bacterial evolution most frequently results from the acquisition of new genes through horizontal gene transfer rather than by the alteration of gene functions through mutations. (It is estimated that as much as 20% of the genome of Escherichia coli originated from horizontal gene transfer.)
There are three mechanisms of horizontal gene transfer in bacteria: transformation, transduction, and conjugation. The most common mechanism for horizontal gene transmission among bacteria, especially from a donor bacterial species to different recipient species, is conjugation. Although bacteria can acquire new genes through transformation and transduction, this is usually a more rare transfer among bacteria of the same species or closely related species.
We will now look at conjugation.
3. Conjugation (def)
Genetic recombination in which there is a transfer of DNA from a living donor bacterium to a living recipient bacterium by cell-to-cell contact. In gram-negative bacteria it typically involves a conjugation or sex pilus (def).
Conjugation is encoded by plasmids. It involves a doner bacterium that contains a conjugative plasmid and a recipient cell that does not. A conjugative plasmid (def) is self-transmissible, in that it possesses all the necessary genes for that plasmid to transmit itself to another bacterium by conjugation. Conjugation genes known as tra genes enable the bacterium to form a mating pair (def) with another organism, while oriT (origin of transfer) genes determine where on the plasmid DNA transfer is initiated. In addition, mobilizable plasmids (def) that lack the tra genes for self-transmissibility but possess the oriT genes for initiation of DNA transfer may also be transferred by conjugation if the bacterium containing them also possesses a conjugative plasmid. The tra genes of the conjugative plasmid enable a mating pair to form and the oriT genes of the mobilizable plasmid enable the DNA to moves through the conjugative bridge (see Fig. 8).
As learned earlier in this unit under "Plasmids and Transposons", transposons ("jumping genes" (def)) are small pieces of DNA that encode enzymes that enable the transposon to move from one DNA location to another, either on the same molecule of DNA or on a different molecule. Transposons may be found as part of a bacterium's nucleoid (conjugative transposons) or in plasmids and are usually between one and twelve genes long. A transposon contains a number of genes, such as those coding for antibiotic resistance or other traits, flanked at both ends by insertion sequences coding for an enzyme called transpoase. Transpoase is the enzyme that catalyzes the cutting and resealing of the DNA during transposition.
Conjugative transposons (def), like conjugative plasmids, carry the genes that enable mating pairs to form for conjugation. Therefore, conjugative transposons also enable mobilizable plasmids and nonconjugative transposons to be transferred to a recipient bacterium during conjugation.
Many conjugative plasmids and conjugative transposons possess rather promiscuous transfer systems that enables them to transfer DNA not only to like species, but also to unrelated species. The ability of bacteria to adapt to new environments as a part of bacterial evolution most frequently results from the acquisition of large DNA sequences from another bacterium by conjugation.
a. General mechanism of transfer of conjugative plasmids by conjugation in gram-negative bacteria
In gram-negative bacteria, the first step in conjugation involves a conjugation pilus (sex pilus or F pilus) on the donor bacterium binding to a recipient bacterium lacking a conjugation pilus. Typically the conjugation pilus retracts or depolymerizes pulling the two bacteria together. A series of membrane proteins coded for by the conjugative plasmid then forms a bridge and an opening between the two bacteria, now called a mating pair.
Using the rolling circle model of DNA replication, a nuclease breaks one strand of the plasmid DNA at the origin of transfer site (oriT) of the plasmid and that nicked strand enters the recipient bacterium. The other strand remains behind in the donor cell. Both the donor and the recipient plasmid strands then make a complementary copy of themselves. Both bacteria now possess the conjugative plasmid. This process is summarized in Figs 5A through 5F).
This is the mechanism by which resistance plasmids (R-plasmids) (def), coding for multiple antibiotic resistance and conjugation pilus formation, are transferred from a donor bacterium to a recipient. This is a big problem in treating opportunistic gram-negative infections such as urinary tract infections, wound infections, pneumonia, and septicemia by such organisms as E. coli, Proteus, Klebsiella, Enterobacter, Serratia, and Pseudomonas, as well as with intestinal infections by organisms like Salmonella and Shigella.
There is also evidence that the conjugation pilus may also serve as a direct channel through which single-stranded DNA may be transferred during conjugation.
b. F+ conjugation (def)
This results in the transfer of an F+ plasmid (def) possessing tra genes coding only for a conjugation pilus and mating pair formation from a donor bacterium to a recipient bacterium. One strand of the F+ plasmid is broken with a nuclease at the origin of transfer (oriT) site of the plasmid. The nicked strand enters the recipient bacterium while the other plasmid strand remains in the donor. Each strand then makes a complementary copy. The recipient then becomes an F+ male and can make a sex pilus (see 6A through 6F).
In addition, mobilizable plasmids (def) that lack the tra genes for self-transmissibility but possess the oriT genes for initiation of DNA transfer, may also be transferred by conjugation. The tra genes of the F+ plasmid enable a mating pair to form and the oriT genes of the mobilizable plasmid enable the DNA to moves through the conjugative bridge (see Fig. 8).
c. Hfr (high frequency recombinant) conjugation (def)
Hfr recombination begins when an F+ plasmid with tra genes coding for mating pair formation inserts or integrates into the nucleoid to form an Hfr bacterium. (A plasmid that is able to integrate into the host nucleoid is called an episome.) A nuclease then breaks one strand of the donor's DNA at the origin of transfer (oriT) location of the inserted F+ plasmid and the nicked strand of the donor DNA begins to enter the recipient bacterium. The remaining non-nicked DNA strand remains in the donor and makes a complementary copy of itself.
The bacterial connection usually breaks before the transfer of the entire chromosome is completed so the remainder of the F+ plasmid seldom enters the recipient. As a result, there is a transfer of some chromosomal DNA, which may be exchanged for a piece of the recipient's DNA through homologous recombination, but not the ability to form a conjugation pilus and mating pairs (see Fig. 7A through 7H).
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