Studying meiosis in mitosis: Activating the meiosis-specific Red1-Hop1-Mek1 complex in mitotic budding yeast cells

In mitosis, sequences on sister chromatids are preferred as DNA repair templates, whereas in meiosis interhomolog-based repair is promoted. The switch of template preference during homologous recombinational (HR) repair of DNA breaks is a defining event in sexual reproduction. This preference is needed to establish linkages between homologous chromosomes that support meiotic chromosome segregation. In budding yeast, a central activity that enforces meiotic interhomolog bias is encoded in a meiosis-specific protein kinase complex, consisting of Red1, Hop1 and Mek1 (i.e., the RHM complex). Activation of Mek1 kinase in meiosis – dictated by complex formation and upstream DNA break-dependent signaling – leads to modification of HR factors and the establishment of interhomolog HR repair bias. How meiotic repair bias is established is a central question with implications for sexual reproduction, genetic diversity and genome stability. Studying the role of the RHM complex in DNA repair is complicated by the fact that Red1 and Hop1 are required for efficient meiotic DNA break formation. Here, we conditionally express RHM components in mitotically-dividing cells to show that these factors can autonomously establish the RHM complex outside of its physiological environment. In vivo analysis is complemented with in vitro biochemical reconstitution to analyze the composition of a Red1-Hop1 subcomplex. The RHM complex can be activated under DNA damaging conditions in mitotically-dividing cells, and activation depends on upstream Mec1 kinase function. We use this system to perform a structure-function analysis of RHM complex formation and Mek1 activation. Finally, we demonstrate that expressing active Mek1 in mitosis leads to rad51Δ-like DNA break sensitivity, suggesting that activation of the RHM complex is sufficient to reconstitute (parts of) its physiological function in mediating HR-based repair. This system should enable querying downstream effects of RHM complex action on DNA repair dynamics and template bias. Human homologs of Red1 and Hop1 are often aberrantly re-expressed in cancer cells. Our system has the potential to inform on (dys)functional effects of these genes on genome stability during human tumorigenesis.