Phosphorylation and the creation of interhomolog bias during meiosis in yeast.

FEATURES One of the fundamental differences between mitotic and meiotic cells is the source of homology used to repair double strand breaks (DSBs). Recombination in vegetative cells occurs preferentially between sister chromatids to repair unexpected DNA damage. 1 In contrast, repair of programmed DSBs in meiotic cells occurs primarily between non-sister chro-matids of homologous chromosomes. 2 The resulting crossovers generate physical connections that promote accurate segregation at the first meiotic division. Mistakes in meiotic chromosome segregation in humans lead to infertility and birth defects. An important question, therefore, is how the bias for recombination between homologs is established during meiosis. The decision of whether to use sister chromatids or homologs as templates for repair is made at the time of strand invasion. Strand invasion is mediated by recombinases, RecA-like proteins that bind to the single stranded ends of resected DSBs to form filaments. 3 The major recombinase in vegetative yeast cells is Rad51. Rad51 activity requires interaction with Rad54, a member of the Swi/Snf family of chromatin remod-eling proteins. Rad54 helps to clear off nucleosomes, denature the recipient DNA duplex to promote strand invasion and remove Rad51 after strand invasion is complete. 4 In meiotic cells, a meiosis-specific recombinase called Dmc1 is also present. Dmc1 functions with a paralog of Rad54 called Rdh54/Tid1. Although both RAD51 and DMC1 are required for interhomolog recombination, a variety of data suggests that Rad51 and Dmc1 are utilized primarily for inter-sister and inter-homolog recombination, respectively. clarify the different roles these two recombinases play during meiosis. The activity of a meiosis-specific kinase called Mek1/Mre4 is necessary to prevent Rad51 from invading sister chromatids, but the substrates of Mek1 responsible for suppressing inter-sister DSB repair were unknown. 6 Niu et al. screened various purified proteins involved in Rad51-mediated recombination for phosphorylation by Mek1 and found that Rad54 is phospho-rylated on threonine 132 both in vitro and in meiotic cells. The negative charge conferred by phosphorylation reduces the affinity of Rad54 for Rad51 and reduces Rad54 stimulation of Rad51's activity. In vivo, the inability to phosphorylate Rad54 suppresses the interhomolog recombina-tion defect of dmc1∆. This suppression, however, is dependent upon Mek1, indicating that Mek1 performs two functions during meiosis: (1) it inhibits Rad51 recombinase activity by decreasing Rad51/ Rad54 complex formation through phos-phorylation of Rad54 and (2) it prevents filaments containing Rad51/Rad54 from invading sister chromatids by phospho-rylation of a second, as yet unknown, protein (Fig. 1). …