HOTSPOTS OF HOMOLOGOUS RECOMBINATION

Homologous recombination occurs at higher than average frequency at an d near hotspots. Hotspots are special nucleotide sequences recognized by proteins that promote, directly or indirectly, a rate limiting step of recombination. This review focuses on two well-studied examples, t he Chi sites of the bacterium Escherichia coli and the M26 site of the fission yeast Schizosaccharomyces pombe. Chi, 5' G-C-T-G-G-T-G-G 3', is recognized by the RecBCD enzyme, which nicks the DNA near Chi and p roduces a 3'-ended single-stranded DNA 'tail'; this tail is a potent s ubstrate for homologous pairing by RecA and single-stranded DNA bindin g proteins. M26, 5' A-T-G-A-C-G-T 3', is recognized by a heterodimeric protein and stimulates, by an as-yet-unknown mechanism, meiotic recom bination at acid near the ade6 gene. Additional hotspots in bacteria, fungi, and mammals enhance recombination directly or indirectly via a variety of mechanisms. Although hotspots are widespread among organism s, the biological role of their localized enhancement of recombination remains a matter of speculation.