loads the DNA onto RecA, which mediates homologous recombination through the classic DNA repair pathway. Regulation of competence In laboratory cultures, natural competence is usually tightly regulated and often triggered by nutritional shortages or adverse conditions. However the specific inducing signals and regulatory machinery are much more variable than the uptake machinery, and little is known about the regulation of competence in the natural environments of these bacteria. Transcription factors have been discovered which regulate competence; an example is sxy (also known as tfoX) which has been found to be regulated in turn by a 5' non-coding RNA element. In bacteria capable of forming spores, conditions inducing sporulation often overlap with those inducing competence. Thus cultures or colonies containing sporulating cells often also contain competent cells. Recent research by Süel et al. has identified an excitable core module of genes which can explain entry into and exit from competence when cellular noise is taken into account. Most competent bacteria are thought to take up all DNA molecules with roughly equal efficiencies, but bacteria in the families Neisseriaceae and Pasteurellaceae preferentially take up DNA fragments containing short DNA sequences, termed DNA uptake sequence (DUS) in Neisseriaceae and uptake signal sequence (USS) in Pasteurellaceae, that are very frequent in their own genomes. Neisserial genomes contain thousands of copies of the preferred sequence GCCGTCTGAA, and Pasteurellacean genomes contain either AAGTGCGGT or ACAAGCGGT. Evolutionary functions and consequences of competence Most proposals made for the primary evolutionary function of natural competence as a part of natural bacterial transformation fall into three categories: (1) the selective advantage of genetic diversity; (2) DNA uptake as a source of nucleotides (DNA as “food”); and (3) the selective advantage of a new strand of DNA to promote homologous recombinational repair of damaged DNA (DNA repair). A secondary suggestion has also been made, noting the occasional advantage of horizontal gene transfer. Hypothesis of genetic diversity Arguments to support genetic diversity as the primary evolutionary function of sex (including bacterial transformation) are given by Barton and Charlesworth . and by Otto and Gerstein. However, the theoretical difficulties associated with the evolution of sex suggest that sex for genetic diversity is problematic. Specifically with respect to bacterial transformation, competence requires the high cost of a global protein synthesis switch, with, for example, more than 16 genes that are switched on only during competence of Streptococcus pneumoniae. However, since bacteria tend to grow