Recombination, mutation and the origin of species.

A major barrier to recombination between bacterial species lies in the mismatch repair system, a complex of proteins that has evolved to proof-read freshly replicated DNA. It now appears that a second system, involving an inducible DNA recombination, repair and mutagenesis pathway, also regulates interspecies recombination, but in a positive way, being required for recombination between Escherichia coli and Salmonella typhimurium. Thus the rate at which newly emerging species of bacteria diverge can be seen as a balance between a permissive state associated with inducible repair and recombination, and the proof-reading of intermediates in the recombination pathway by the mismatch correction system.