The fission yeast Schizosaccharomyces pombe shows bipolar growth and is a convenient model for studying cell polarity and polar growth. This paper shows that the related Schiz, japonicus var. japonicus can switch to unipolar growth and can exist in both yeast and mycelial phases. On solid media, the yeast phase is unstable and prone to switch to the mycelial form, which shows unipolar growth by tip elongation. The hyphae can colonize the body of the substrate (true mycelium) or just its surface (pseudo-mycelium). The yeast-tomycelium transition and the growth of the mycelium are regulated by a nutritional gradient and are associated with extensive vacuolation. The mycelium can convert into arthroconidia or return to the yeast phase in response to environmental changes. These environmentally controlled morphological transitions make Schiz. japonicus var. japonicus an attractive model for the investigation of cell polarity and morphogenesis.

The fission yeast Mcs6 -Mcs2-Pmh1 complex, homologous to metazoan Cdk7-cyclin H-Mat1, has dual functions in cell division and transcription: as a partially redundant cyclin-dependent kinase (CDK)-activating kinase (CAK) that phosphorylates the major cell cycle CDK, Cdc2, on Thr-167; and as the RNA polymerase (Pol) II carboxyl-terminal domain (CTD) kinase associated with transcription factor (TF) IIH. We analyzed conditional mutants of mcs6 and pmh1, which activate Cdc2 normally but cannot complete cell division at restrictive temperature and arrest with decreased CTD phosphorylation. Transcriptional profiling by microarray hybridization revealed only modest effects on global gene expression: a one-third reduction in a severe mcs6 mutant after prolonged incubation at 36°C. In contrast, a small subset of transcripts (ϳ5%) decreased by more than twofold after Mcs6 complex function was compromised. The signature of repressed genes overlapped significantly with those of cell separation mutants sep10 and sep15. Sep10, a component of the Pol II Mediator complex, becomes essential in mcs6 or pmh1 mutant backgrounds. Moreover, transcripts dependent on the forkhead transcription factor Sep1, which are expressed coordinately during mitosis, were repressed in Mcs6 complex mutants, and Mcs6 also interacts genetically with Sep1. Thus, the Mcs6 complex, a direct activator of Cdc2, also influences the cell cycle transcriptional program, possibly through its TFIIH-associated kinase function. INTRODUCTIONCyclin-dependent kinases (CDKs) play a central role in driving cell division in eukaryotic organisms and also perform essential, conserved functions in the transcription cycle of RNA polymerase (Pol) II (reviewed by Morgan, 1997). Whereas most CDKs can be classified as either cell cycle or transcriptional regulators based on a preeminent physiological function, the metazoan Cdk7 complex is essential in both cell division and gene expression, as the CDK-activating kinase (CAK) and as a component of the general transcription factor IIH (TFIIH) (reviewed by Harper and Elledge, 1998). To execute its dual functions in vivo, Cdk7 has evolved distinct substrate specificities for the activation segment (T-loop) of CDKs and the carboxy-terminal domain (CTD) of the Pol II large subunit, and mechanisms to allow their independent regulation (Garrett et al., 2001;Larochelle et al., 2001). Less clear is how (or whether) the Cdk7 complex serves to coordinate cell division with gene expression.To address this question genetically, we turned to the fission yeast Schizosaccharomyces pombe, which also relies, in part, on a dual-function CDK complex to activate CDKs and phosphorylate Pol II (Buck et al., 1995;Damagnez et al., 1995;Hermand et al., 1998;Lee et al., 1999;Saiz and Fisher, 2002). The orthologue of Cdk7 in S. pombe is Mcs6, which associates with the cyclin Mcs2 and the RING-finger protein Pmh1. Both mcs6 and mcs2 were identified in genetic screens for positive regulators of Cdc2 (S. pombe Cdk1), the major cell cycle CDK (Molz et al., 1989...

Wine strains of Saccharomyces cerevisiae are usually homothallic diploids and show chromosome length polymorphism. We describe the analysis of a heterothallic strain heterozygous for the mating types a and alpha. Surveying cultures of the strain, we found a remarkable degree of heterogeneity in ploidy and in electrophoretic karyotype. The CHEF analysis of tetrads and dyads revealed an enormous variability of band patterns hampering the analysis of chromosome segregation. We propose that the instability of ploidy and chromosome polymorphism might be due to heterothallism that precludes the process "genome renewal" (MORTIMER et al. 1994) by selfdiploidization of spore clones.

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