Natural Transformation, Recombination, and Repair

Abstract

Three mechanisms of horizontal gene transfer are commonly observed: natural transformation, conjugation, and transduction. Natural transformation and other mechanisms of horizontal gene transfer are dependent on DNA recombination. The 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. The number of different systems seems to vary considerably between organisms, although this does not necessarily imply altered mutation rates. Natural transformation of Helicobacter pylori, together with DNA recombination and repair, is discussed in this chapter to underline the interdependence of these cellular functions. Natural transformation competence in most gram-negative bacteria is associated with production of type IV pili. The importance of recombination for natural transformation competence is mentioned, but (homologous) recombination is also possible during all other states of transient or permanent diploidy, i.e., during conjugation or transduction or after DNA replication. Branch migration and junction resolution can also be performed by RecG, but both systems may have different functions in (recombinational) DNA repair. The resolution of some Holliday junctions during recombinational repair results in the formation of chromosomal dimers, which have to be resolved by the XerCD site-specific recombinases found in many organisms.