No abstract

An essential feature of meiosis is interhomolog recombination whereby a significant fraction of the programmed meiotic double-strand breaks (DSBs) is repaired using an intact homologous non-sister chromatid rather than a sister. Involvement of Mec1 and Tel1, the budding yeast homologs of the mammalian ATR and ATM kinases, in meiotic interhomlog bias has been implicated, but the mechanism remains elusive. Here, we demonstrate that Mec1 and Tel1 promote meiotic interhomolog recombination by targeting the axial element protein Hop1. Without Mec1/Tel1 phosphorylation of Hop1, meiotic DSBs are rapidly repaired via a Dmc1-independent intersister repair pathway, resulting in diminished interhomolog crossing-over leading to spore lethality. We find that Mec1/Tel1-mediated phosphorylation of Hop1 is required for activation of Mek1, a meiotic paralogue of the DNA-damage effector kinase, Rad53p/CHK2. Thus, Hop1 is a meiosis-specific adaptor protein of the Mec1/Tel1 signaling pathway that ensures interhomolog recombination by preventing Dmc1-independent repair of meiotic DSBs.

No abstract

Recent studies in Saccharomyces cerevisiae by using global approaches have significantly enhanced our knowledge of the components involved in the transcriptional regulation of the cell cycle. The Mcm1p-Fkh2p complex, in combination with the coactivator Ndd1p, plays an important role in the cell cycle-dependent expression of the CLB2 gene cluster during the G2 and M phases ([4-7]; see [8-10]for reviews). Fkh2p is phosphorylated in a cell cycle-dependent manner, and peak phosphorylation occurs coincidentally with maximal expression of Mcm1p-Fkh2p-dependent gene expression. However, the mechanism by which this complex is activated in a cell cycle-dependent manner is unknown. Here, we demonstrate that the forkhead-associated (FHA) domain of Fkh2p directs cell cycle-regulated transcription and that the activity of this domain is dependent on the coactivator Ndd1p. Ndd1p was found to be phosphorylated in a cell cycle-dependent manner by Cdc28p-Clb2p, and, importantly, this phosphorylation event promotes interactions between Ndd1p and the FHA domain of Fkh2p. Furthermore, mutation of the FHA domain blocks these phosphorylation-dependent interactions and abolishes transcriptional activity. Our data therefore link the transcriptional activity of the FHA domain with cell cycle-dependent phosphorylation of the coactivator Ndd1p and reveal a mechanism that permits precise temporal activation of the Mcm1p-Fkh2p complex.

The Cdc5 protein of budding yeast is a polo-like kinase that has multiple roles in mitosis including control of the mitotic exit network (MEN). MEN activity brings about loss of mitotic kinase activity so that the mitotic spindle is disassembled and cytokinesis can proceed. Activity of the MEN is regulated by a small GTPase, Tem1, which in turn is controlled by a two-component GTPase-activating protein (GAP) formed by Bfa1 and Bub2. Bfa1 has been identified as a regulatory target of Cdc5 but there are conflicting deductions from indirect in vivo assays as to whether phosphorylation inhibits or stimulates Bfa1 activity. To resolve this question, we have used direct in vitro assays to observe the effects of phosphorylation on Bfa1 activity. We show that when Bfa1 is phosphorylated by Cdc5, its GAP activity with Bub2 is inhibited although its ability to interact with Tem1 is unaffected. Thus, in vivo inactivation of Bfa1-Bub2 by Cdc5 would have a positive regulatory effect by increasing levels of Tem1-GTP so stimulating exit from mitosis.