Natural Transformation, Recombination, and Repair

In: Helicobacter pylori: Physiology and Genetics. Washington (DC): ASM Press; 2001. Chapter 22.


Bacteria have obtained a significant proportion of their genetic diversity through the acquisition of sequences from related organisms. Horizontal gene transfer produces extremely dynamic genomes in which substantial amounts of DNA are introduced into and deleted from the chromosome. These lateral transfers have effectively changed the ecological and pathogenic character of bacterial species. Three mechanisms of horizontal gene transfer are commonly observed: natural transformation, conjugation, and transduction. Although bacteriophages have been described in Helicobacter pylori (21), there is currently no evidence that transduction really occurs in H. pylori. A DNase-insensitive DNA transfer has been described (31), but a proof of conjugative transfer of plasmids between H. pylori is still lacking. Natural transformation competence was described for the first time in H. pylori a number of years ago (39), and several groups are studying the genes involved in this phenomenon to understand its underlying mechanisms.

Natural transformation and other mechanisms of horizontal gene transfer are dependent on DNA recombination. Exogenous DNA taken up by H. pylori must either be integrated into the chromosome by homologous (or site-specific) recombination or replicated as a plasmid, which might also rely on recombination processes for recircularization (4). Homologous recombination is thus a prerequisite for microbial evolution and genome plasticity, and at the same time, it helps to maintain genetic barriers between species by selecting for DNA with sufficient homology (35). Moreover, recombination is an important cellular function that accompanies DNA replication. Replication forks encountering double-strand breaks or gaps in single-stranded DNA result in stalling of replication. Such lesions are repaired by homologous recombination, which is also essential for reinitiation of the replication process (for a review, see reference 30).

This latter mechanism is probably the most important function in postreplicative DNA repair. All organisms respond to the continuous damaging of their DNA by exogenous or endogenous factors with several, possibly redundant, systems of DNA repair. Besides recombinational repair, more specialized repair procedures, not dependent on replication, are employed as well. The number of different systems seems to vary considerably between organisms, although this does not necessarily imply altered mutation rates.

Natural transformation of H. pylori, together with DNA recombination and repair, is discussed in this chapter to underline the interdependence of these cellular functions.

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