The Mei5-Sae3 protein complex mediates Dmc1 activity in Saccharomyces cerevisiae.

During homologous recombination, a number of proteins cooperate to catalyze the loading of recombinases onto single-stranded DNA. Single-stranded DNA-binding proteins stimulate recombination by coating single-stranded DNA and keeping it free of secondary structure; however, in order for recombinases to load on single-stranded-DNA-binding protein-coated DNA, the activity of a class of proteins known as recombination mediators is required. Mediator proteins coordinate the handoff of single-stranded DNA from single-stranded DNA-binding protein to recombinase. Here we show that a complex of Mei5 and Sae3 from Saccharomyces cerevisiae preferentially binds single-stranded DNA and relieves the inhibition of the strand assimilation and DNA binding abilities of the meiotic recombinase Dmc1 imposed by the single-stranded DNA-binding protein replication protein A. Additionally, we demonstrate the physical interaction of Mei5-Sae3 with replication protein A. Our results, together with previous in vivo studies, indicate that Mei5-Sae3 is a mediator of Dmc1 assembly during meiotic recombination in S. cerevisiae.