13.4 Gene Transfer in Bacteria

Discuss the role of horizontal gene transfer in the evolution of bacteria.

Bacteria evolved rapidly due to horizontal gene transfer of genes, including packets of genes called pathogenicity islands, between bacterial species.

Compare the three mechanisms of gene transfer in bacteria: transformation, conjugation, and transduction.

In transformation, donor DNA in the growth medium enters a recipient cell. Conjugation depends on direct cell-to-cell contact between a donor carrying a conjugative plasmid (e.g., the F plasmid) or integrated conjugative element (as in Hfr strains), and a recipient lacking such an element. In transduction, bacteria DNA packaged into the protein coat of a phage is the vehicle for gene transfer.

Explain how each of these three methods of gene transfer can be used to map bacterial genes.

To map genes by conjugation, an interrupted mating procedure is used. During the conjugation process, an Hfr strain is mixed with an F- strain. The two strains must have different genotypes and must remain in physical contact for the transfer to occur. At regular intervals, the conjugation process is interrupted. The chromosomal transfer from the Hfr strain always begins with a portion of the integrated F factor and proceeds in a linear fashion. To transfer the entire chromosome would require approximately 100 minutes. The time required for individual genes to be transferred is relative to their position on the chromosome and the direction of transfer initiated by the F factor. Gene distances are typically mapped in minutes of conjugation. The genes that are transferred by conjugation to the recipient must be incorporated into the recipient's chromosome by recombination to be expressed.transformation is used to map bacterial genes. In transformation, the relative frequency at which pairs of genes are transferred or cotransformed indicates the distance between the two genes. Closer gene pairs are cotransformed more frequently. As was the case with conjugation, the donor DNA must recombine into the recipient cell's chromosome. Physical contact of the donor and the recipient cells is not needed. The recipient cell uptakes the DNA directly from the environment. Therefore, the DNA from the donor strain has to be isolated and broken up before transformation can take place. A viral vector is needed for the transfer of DNA by transduction. DNA from the donor cell is packaged into a viral protein coat. The viral particle containing the bacterial donor DNA then infects another bacterial cell or the recipient. The donor bacterial DNA is incorporated into the recipient cell's chromosome by recombination. Only genes that are close together on the bacterial chromosome can be cotransduced. Therefore, the rate of cotransduction, like the rate of cotransformation, gives an indication of the physical distances between genes on the chromosome