Positive selection and recombination: major molecular mechanisms in colicin diversification.

Colicins, a family of antimicrobial proteins produced by Escherichia coli, are one of the best characterized microbial systems for studying processes of molecular diversification. Recent studies employing DNA sequence comparisons and experimental evolution suggest that positive selection and recombination play dominant roles in colicin diversification. Recombination between distantly related colicins has repeatedly generated novel classes of colicins, while positive selection for novel colicin immunity systems produces further diversity among closely related colicins. Together, these forces have resulted in a surprisingly large and diverse class of antimicrobials. Colicins are thought to play an important role in the invasion of bacteria into novel habitats.