Analysis of strains lacking the NDT80 gene revealed that ndt80Δ cells arrest in prophase I, whereas ndt80Δ ama1Δ cells form an MI spindle, indicating activation of M-Cdk1. Whereas deletion of AMA1 blocks the degradation of Ndt80-GFP at anaphase II, it does not affect the appearance of Ndt80-GFP ( Figure S1B), which is frequently preceded by premature SPB separation in the mutant cells (see also Figure 2C). To investigate whether Ndt80 expression is altered in ama1Δ cells, we imaged Ndt80-GFP together with Cdc7-RFP or Cnm67-RFP. In a WT meiosis, spindle formation depends on the transcription factor Ndt80. Here, we have addressed this question in budding yeast. How mitotic M phase controls are prevented from interfering with the long meiotic prophase is unclear. Indeed, different transcription factors mediate entry into mitotic and meiotic M phase in yeast. Mitotic cells activate Cdk1-Clb soon after S phase, whereas meiotic cells do not activate Cdk1-Clb until after a long prophase during which homologous chromosomes (homologs) undergo recombination. Although cyclin-dependent kinase 1 bound to cyclin B (Cdk1-Clb) is universally required for spindle formation, which marks entry into M phase, the timing of this event differs dramatically in mitosis and meiosis. Meiosis, by contrast, creates haploid gametes from diploid germ cells because DNA replication is followed by two consecutive M phases during which dyad chromosomes segregate in meiosis I (MI) and chromatids disjoin in meiosis II (MII). Mitotic divisions generate genetically identical daughter cells because DNA replication alternates with an M phase in which sister chromatids segregate. Most eukaryotic genomes encode the machinery to orchestrate two types of cell division. Thus, control of prophase I by meiotic mechanisms depends on the suppression of the alternative, mitotic mechanisms by a meiosis-specific form of the APC/C. ama1Δ mutants exit from prophase I prematurely and independently of the RC, which results in recombination defects and chromosome missegregation.
Diagram of prophase Activator#
We show that an extended prophase I additionally requires the suppression of latent, mitotic cell-cycle controls by the anaphase-promoting complex (APC/C) and its meiosis-specific activator Ama1, which trigger the degradation of M phase regulators and Ndd1, a subunit of a mitotic transcription factor. Exit from prophase I is controlled by the recombination checkpoint (RC), which, in yeast, represses the meiosis-specific transcription factor Ndt80 required for the expression of B-type cyclins and other M phase regulators.
Whereas proliferating cells enter M phase shortly after DNA replication, the first M phase of meiosis is preceded by an extended prophase in which homologous chromosomes undergo recombination.
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