Competence for genetic transformation is a physiological state that enables the uptake of exogenous DNA. Although competence is widespread in nature (Lorenz and Wackernagel, 1994), it appears to be a variable phenotype because natural isolates of a given species may or may not be transformable, and genome sequencing has revealed the presence of competence genes in isolates that are not known to be transformable.
WHAT USE IS COMPETENCE?
In this chapter, we consider three disputed hypotheses that have been advanced for the biological role of competence: DNA uptake for new genetic information, DNA uptake for repair, and DNA uptake for nutrition.
DNA for genetic diversity
The evolutionary pressure for the maintenance of competence genes may be explained by the advantages gained from the acquisition of fitness-enhancing genes; competence may expand the repertoire of genes available to improve the chances of survival in harsh conditions. Several examples of the acquisition by transformation of new genes that confer a selective advantage have been suggested. For instance, in Neisseria gonorrhoeae, in which transformation is the only known mode of DNA transfer, the expression of new allelic variants may facilitate antigenic variation, which presumably plays a role in the evasion of the host immune response. Thus, although new pilin variants may be formed by intracellular recombination between a silent gene segment and an expressed pilin gene, the intercellular exchange of pilin alleles may also occur by transformation (Gibbs et al., 1989; Seifert et al., 1988).