The 3rd Exon of the Budding Yeast Meiotic Recombination Gene HOP2 is Required for Calcium-dependent and Recombinase Dmc1-specific Stimulation of Homologous Strand Assimilation [DNA and Chromosomes]

May 5th, 2014 by Chan, Y.-L., Brown, M. S., Qin, D., Handa, N., Bishop, D. K.

During meiosis in Saccharomyces cerevisiae, the HOP2 and MND1 genes are essential for recombination. A previous biochemical study showed budding yeast Hop2-Mnd1 stimulated the activity of the meiosis-specific strand exchange protein ScDmc1 only 3-fold, whereas analogous studies using mammalian homologs showed >30-fold stimulation. The HOP2 gene was recently discovered to contain a second intron that lies near the 3 prime end. We show both HOP2 introns are efficiently spliced during meiosis, forming a predominant transcript that codes for a protein with a C-terminal sequence different from that of the previously studied version of the protein. Using the newly identified HOP2 open reading frame to direct synthesis of wild type Hop2 protein, we show that the Hop2-Mnd1 heterodimer stimulates Dmc1 D-loop activity up to 30-fold, similar to the activity of mammalian Hop2-Mnd1. ScHop2-Mnd1 stimulates ScDmc1 activity in the presence of physiological (micromolar) concentrations of Ca2+ ions, as long as Mg2+ is also present at physiological concentrations, leading us to hypothesize that ScDmc1 protomers bind both cations in the active Dmc1 filament. Co-factor requirements and order of addition experiments suggest that Hop2-Mnd1-mediated stimulation of Dmc1 involves a process that follows formation of functional Dmc1-ssDNA filaments. In dramatic contrast to mammalian orthologs, the stimulatory activity of budding yeast Hop2-Mnd1 appears to be specific to Dmc1; we observed no Hop2-Mnd1-mediated stimulation of the other budding yeast strand exchange protein Rad51. Together, these results support previous genetic experiments indicating that Hop2-Mnd1 specifically stimulates Dmc1 during meiotic recombination in budding yeast.
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