Functional analysis of Cdc7 kinase activity reveals multiple roles in budding yeast meiosis

Abstract

Cdc7, a highly conserved serine/threonine kinase, is essential for DNA replication in mitotically dividing cells. Previous work using temperature-sensitive mutants of CDC7 demonstrated a requirement for this kinase in meiotic recombination and progression, but the mechanisms by which Cdc7 promotes these processes were unknown. Using a chemical genetic approach, my work demonstrated that Cdc7 kinase activity allows entry into the meiotic divisions by enabling expression of NDT80. NDT80 is a meiosis-specific transcriptional activator required for expression of middle sporulation genes involved in exit from pachytene, meiotic progression and spore formation. In addition, this work revealed a previously unknown role for CDC7 in the mono-orientation of kinetochores at the first meiotic division. NDT80 is expressed in two steps: first, transcription depends on activation by Ime1 and removal of Sum1-mediated repression. Second, the resulting Ndt80 protein upregulates it own expression, creating a positive autoregulatory loop. My work shows that Cdc7 kinase activity is required to relieve Sum1-mediated repression to promote Ime1-driven NDT80 expression, possibly through removal of histone deacetylase activity conferred by the Hst1/Rfm1 complex that associates with Sum1. At Meiosis I, sister chromatids co-segregate to the same spindle pole, thereby separating homologous chromosomes in a reductional division. This process requires the localization and maintenance of the monopolin complex at kinetochores during Metaphase I. Genetic and cytological analyses indicate that Cdc7 kinase activity is essential to recruit monopolin onto sister kinetochores to allow reductional segregation at Meiosis I. The semisynthetic epitope approach was combined with an analog sensitive version of Cdc7 (Cdc7-as) to identify meiosis-specific substrates. Cdc7-as immunoprecipitated from vegetative and meiotic cells was used in in vitro kinase assays and meiosis-specific substrates of the kinase were detected. Identification of these meiotic substrates may provide insights into how Cdc7 coordinates different meiotic processes in budding yeast and metazoans.