Chapter09_SSM - 65781_CH09_175_197.qxd 12:59 PM Page 175...

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Chapter 9: Genetics of Bacteria and Their Viruses Chapter Summary Bacterial cells often contain plasmids—nonessential DNA molecules that replicate and are transmitted to progeny cells. Some plasmids, like the F (fertility) factor, are conjugative: They contain genes that make possible their transfer between cells through a conjugation structure. Other plasmids are nonconjugative, but through homologous recombination they can form and cointegrate with a conjugative plasmid and be transferred by a sort of hitchhiking. Most bacteria also possess transposable elements, which are capable of moving from one part of the DNA to another. Transposition does not require sequence homology. Bacterial insertion sequences (IS) are small transposable elements that code for their own transposition and that have inverted repeat sequences at the ends. Some larger transposons have a composite structure consisting of a central region, often containing one or more antibiotic-resistance genes, flanked by two IS sequences in inverted orientation. DNA sequences called integrons encode a site-specific recombinase (integrase) and can capture circular cassettes of DNA by site-specific recombination between an attI site in the integron and an attC site in the cassette. Accumulation of mobile DNA elements can result in multiple-drug resistant R plasmids or the incorporation of pathogenicity islands (linked groups of genes associated with disease) into the bacterial chromosome. DNA can be transferred between bacteria in three ways: transformation, conjugation, and transduction. In transformation, free DNA molecules, obtained from donor cells, are taken up by recipient cells; by a recombinational mechanism, a single-stranded segment becomes integrated into the recipient chromosome, replacing a homologous segment. In conjugation, donor and recipient cells pair, and a single strand of DNA is transferred by rolling-circle replication from the donor cell to the recipient. Transfer is mediated by cointegrated formation between the F plasmid and the bacterial chromosome, creating an Hfr (high frequency of recombination) donor strain; transfer begins at the replication origin of F and proceeds linearly. Only part of the donor chromosome is usually transferred, and it can be maintained in the recipient only after an exchange event. About 100 minutes are required to transfer the entire E. coli chromosome, but the mating cells usually break apart before transfer is complete. The times at which donor markers first enter recipient cells—the times of 175
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176 entry—can be arranged in order, yielding a genetic map that is circular because of the multiple sites at which an F plasmid integrates into the bacterial chromosome. Occasionally, F is excised from the chromo- some in an Hfr cell; aberrant excision, in which one cut is made at the end of F and the other cut is made in the chromosome, gives rise to an F plasmid, which carries and can transfer bacterial genes. The term epi- some
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Chapter09_SSM - 65781_CH09_175_197.qxd 12:59 PM Page 175...

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