Saccharomyces cerevisiae Mer3 Is a DNA Helicase Involved in Meiotic Crossing Over

Abstract

Crossing over is regulated to occur at least once per each pair of homologous chromosomes during meiotic prophase to ensure proper segregation of chromosomes at the first meiotic division. In a mer3 deletion mutant of Saccharomyces cerevisiae, crossing over is decreased, and the distribution of the crossovers that occur is random. The predicted Mer3 protein contains seven motifs characteristic of the DExH box type of DNA/RNA helicases. The mer3G166D and the mer3K167A mutation, amino acid substitutions of conserved residues in a putative nucleotide-binding domain of the helicase motifs caused a defect in the transition of meiosis-specific double-strand breaks to later intermediates, decreased crossing over, and reduced crossover interference. The purified Mer3 protein was found to have DNA helicase activity. This helicase activity was reduced by the mer3GD mutation to <1% of the wild-type activity, even though binding of the mutant protein to single- and double-strand DNA was unaffected. The mer3KA mutation eliminated the ATPase activity of the wild-type protein. These results demonstrate that Mer3 is a DNA helicase that functions in meiotic crossing over.

We are grateful to Hernan Flores-Rozas, Jason Kahana, and Albrecht Clement for advice on protein purification and John Weger and Jill Green for DNA sequencing. We also thank Neelam Amin, Ruchira Das Gupta, Martin Hess, Patrick Lau, Kyungjae Myung, and Hernan Flores-Rozas for comments on the manuscript.

This work was supported by National Institutes of Health grant GM26017 to R.D.K. T.N. was supported by the Japan Society for the promotion of Science and Human Frontier Science Program.