ISOLATION AND CHARACTERIZATION OF RAD51 ORTHOLOGS FROM COPRINUS-CINEREUS AND LYCOPERSICON-ESCULENTUM, AND PHYLOGENETIC ANALYSIS OF EUKARYOTIC RECA HOMOLOGS

In eubacteria, the recA gene has long been recognized as essential for homologous recombination and DNA repair. Recent work has identified r ecA homologs in archaebacteria and eukaryotes, thus emphasizing the un iversal role this gene plays in DNA metabolism. We have isolated and c haracterized two new recA homologs. one from the basidiomycete Coprinu s cinereus and the other from the angiosperm Lycopersicon esculentum. Like the RAD51 gene of Saccharomyces cerevisiae, the Coprinus gene is highly induced by gamma irradiation and during meiosis. Phylogenetic a nalyses of eukaryotic recA homologs reveal a gene duplication early in eukaryotic evolution which gave rise to two putatively monophyletic g roups of recA-like genes. One group of 11 characterized genes, designa ted the rad51 group, is orthologous to the Saccharomyces RAD51 gene an d also contains the Coprinus and Lycopersicon genes. The other group o f seven genes, designated the dmc1 group, is orthologous to the Saccha romyces DMC1 gene. Sequence comparisons and phylogenetic analysis reve al extensive lineage- and gene-specific differences in rates of RecA p rotein evolution. Dmc1 consistently evolves faster than Rad51, and fun gal proteins of both types, especially those of Saccharomyces, change rapidly, particularly in comparison to the slowly evolving vertebrate proteins. The Drosophila Rad51 protein has undergone remarkably rapid sequence divergence.