Genetic Recombination: An Overview

I. MICROBIAL GENETICS

G. Horizontal Gene Transfer in Bacteria

1. An Overview

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Horizontal Gene Transfer in Bacteria (def)

Bacteria are able to respond to selective pressures and adapt to new environments by acquiring new genetic traits as a result of mutation (def), a modification of gene function within a bacterium, and as a result of horizontal gene transfer (def), the acquisition of new genes from other bacteria.

Mutation, mentioned in the previous section, occurs relatively slowly. The normal mutation rate in nature is in the range of 10^-6 to 10^-9 per nucleotide per bacterial generation, although when bacterial populations are under stress, they can greatly increase their mutation rate. Furthermore, most mutations are harmful to the bacterium.

Review of Mutation from Unit 6
Horizontal gene transfer, on the other hand, enables bacteria to respond and adapt to their environment much more rapidly by acquiring large DNA sequences from another bacterium in a single transfer.

In this section we will look at horizontal gene transfer.

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.)

Horizontal gene transfer is able to cause rather large-scale changes in a bacterial genome. For example, certain bacteria contain multiple virulence genes called pathogenicity islands (def) that are located on large, unstable regions of the bacterial genome. These pathogenicity islands can be transmitted to other bacteria by horizontal gene transfer. However, if these transferred genes provide no selective advantage to the bacteria that acquire them, they are usually lost by deletion. In this way the size of the bacterium's genome can remain approximately the same size over time.

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.

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