11.1: What counts as sex in prokaryotes

Conjugation is a major pathway for horizontal gene transfer in bacteria.^[405] In contrast to transformation, conjugation “forces” DNA into what may be a reluctant recipient cell. In the process of conjugation, we start by distinguishing between two types of bacterial cells (of the same species). F⁺ cells contain a DNA sequence known as the fertility (or

sex) factor F, cells without the F factor are referred to as F^– cells. The F factor can exist independently of the host chromosome as a plasmid. Discovered by Esther Lederberg (1922–2006), the F plasmid can be integrated into the host chromosome. When integrated the cells become what are known as high frequency recombination (Hfr) cells. The ~100 kilobase F plasmid contains about 100 genes that encode the proteins needed to transfer a single-stranded copy of its DNA into a cell that lacks an F-plasmid.^[406] F plasmids typically contain genes that encode an addiction system (discussed earlier). Such systems encode a stable toxin and an unstable (rapidly degraded) anti-toxin. Once the plasmid enters a cell, both toxin and anti-toxin proteins are synthesized. If the plasmid is lost, the cell dies because of the anti-toxin disappearing before the toxin, leading to toxin activation and cell death.

Here we consider a population of F^– cells into which we insert an F⁺ cell. The F-plasmid contains two distinct origins of replication - one, known as oriV, is involved in normal plasmid replication during cell growth and division. The second, known as oriT, is involved in generating the single stranded DNA molecule that can be transferred into a F^– recipient cell. To initiate conjugation, the F⁺ cell makes a physical (conjugation) bridge, known as a pillus, that connects to the F^– cell (→). A single stranded copy of the F plasmid is synthesized and transferred through the pillus into the recipient F^– cell. Subsequent DNA synthesis generates a double-stranded copy of the F-plasmid in the recipient cell, while the donor cell retains the original plasmid.

If the F-plasmids integrates into the host cell genome, the cell becomes an Hfr cell (←). Integration of the F-plasmid can occur at various points along the host chromosome. As with the free plasmids, the integrated F-plasmid can initiate (at its oriT site) the transfer of its own as well as linked host genes into a F^– cell. The amount of DNA transferred will be determined largely by how long the bridge between the cells remains intact. In E. coli it takes ~100 minutes to transfer the entire donor chromosome from an Hfr to an F^– cell. Once inside the F^– cell, the transferred donor DNA is integrated (by homologous recombination) into the recipient’s chromosome, replacing the recipient’s versions of the genes transferred (a process to which we will return). Using Hfr strains carrying different alleles of genes associated with recognizable phenotypes, and by controlling the duration of conjugation by breaking the conjugation bridge by shearing the cells in a kitchen blender, experimenters were able to determined the order of genes along the chromosome. The result was the discovery that related organisms often had the same genes arranged in the same order.^[407] The typical drawing of the circular bacterial chromosome is like a clock going from 0 to 100, with the genes placed in their respective positions, based on the time it takes to transfer them in minutes (→).

If the entire F-plasmid sequence is transferred, the original F^– cell becomes an Hfr cell. If a Hfr cell loses the F-plasmid sequence, it reverts to a F^– state. The end result of the conjugation process is similar to that obtained in sexual reproduction in eukaryotes, namely the original F^– cell now has a genome derived in part from itself and from a“donor” Hfr cell. The outcome of an Hfr/F^– cell interaction can lead to a cell with a different set of alleles than either of the “parental” cells, this process is often referred to as bacterial sex, although it is mechanistically quite distinct from sexual reproduction in eukaryotes. Versions of this process are involved in the transfer of plasmids from cell to cell within a community.^[408]

All plasmids contain an “origin of replication”. "Low copy number" plasmids exist in one to two copies per cell, while "high copy number" plasmids may be present in as many as ~700 copies per cell.^[409] Plasmid copy number is determined in large part by their origin of replication sequences. Plasmids can encode genes responsible for antibiotic resistance; the rapid dispersion of the antibiotic resistance phenotype is a cause of increasing concern.^[410] Many plasmids, also known as mobile genetic elements, are selfish, that is, their presence may not directly benefit that cell and their loss may result in the death of the host cell, due to the presence of an addiction module.

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