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Cdc37 is a molecular chaperone that interacts with a range of clients and co-chaperones, forming various high molecular mass complexes. Cdc37 sequence homology among species is low. High homology between yeast and metazoan proteins is restricted to the extreme N-terminal region, which is known to bind clients that are predominantly protein kinases. We show that despite the low homology, both Saccharomyces cerevisiae and human Cdc37 are able to substitute for the Schizosaccharomyces pombe protein in a strain deleted for the endogenous cdc37 gene. Expression of a construct consisting of only the N-terminal domain of S. pombe Cdc37, lacking the postulated heat-shock protein (Hsp) 90-binding and homodimerization domains, can also sustain cellular viability, indicating that Cdc37 dimerization and interactions with the cochaperone Hsp90 may not be essential for Cdc37 function in S. pombe. Biochemical investigations showed that a small proportion of total cellular Cdc37 occurs in a high molecular mass complex that also contains Hsp90. These data indicate that the N-terminal domain of Cdc37 carries out essential functions independently of the Hsp90-binding domain and dimerization of the chaperone itself.

Highly purified DNA polymerase δ from the fission yeast Schizosaccharomyces pombe is a complex of at least four distinct subunits. Genes encoding three of these (pol3 + /cdc6 + , cdc1 + and cdc27 + ) have been characterised previously. Here we describe the isolation and characterisation of cdm1 + , the gene encoding the smallest (22kDa) subunit of the Pol δ complex. Over-expression of cdm1 + , which encodes a 160 amino-acid protein with no significant sequence similarity to proteins in current databases, is able to rescue cells carrying temperature-sensitive mutations in either pol3 + /cdc6 + , cdc1 + or cdc27 + . Cells deleted for cdm1 + are viable, indicating that cdm1 + is non-essential for mitotic growth, and are no more sensitive to a variety of DNA replication inhibitors and DNA damaging agents than are wild-type cells. In addition, over-expression of cdm1 + suppresses the temperature-sensitive cdc24-M38 mutant suggesting that cdc24 + may also have a role in DNA polymerase δ function.

The Schizosaccharomyces pombe wis1 + gene is essential for cell survival under stress conditions. The MAPKK homologue Wis1 is required for activation of the MAPK homologue Spc1, and integrity of the Wis1–Spc1 pathway is required for survival in extreme conditions of heat, osmolarity, oxidation or limited nutrition. We show here that Wis4, a protein kinase of a new MAPKKK class, phosphorylates Wis1 in vitro and activates it in vivo . Win1 is also required for full activation of Wis1, and Win1 rather than Wis4 mediates the osmotic stress signal. Surprisingly, the pathway can still be activated by heat or oxidative stress independently of the phosphorylation of two conserved Wis1 residues. Evidence is presented that the Pyp1 protein tyrosine phosphatase, which dephosphorylates Spc1, is central to this alternative activation mechanism.

In this paper we describe properties of the cdc10-C4 mutant of the fission yeast Schizosaccharomyces pombe. The cdc10+ gene encodes a component of the DSC1Sp/MBF transcription complex, which is required for cell-cycle regulated expression at G1-S of several genes via cis-acting MCB (MIuI cell cycle box) elements. At permissive temperatures cdc10-C4 causes expression of MCB-regulated genes through the whole cell cycle, which in asynchronously dividing cells is manifested in overall higher expression levels. This overexpression phenotype is cold sensitive: in cdc10-C4 cells, MCB genes are expressed offprogressively higher levels at lower temperatures. In heterozygous cdc10-C4/cdc10+ diploid strains, MCB-regulated genes are not overexpressed, suggesting that loss, rather than alteration, of function of the cdc10-C4 gene product is the reason for unregulated target gene expression. Consistent with this, the cdc10-C4 mutant allele is known to encode a truncated protein. We have also overexpressed the region of the cdc10 protein absent in cdc10-C4 under the control of an inducible promoter. This induces a G1 delay, and additionally causes a reduction of the overexpression of MCB genes in cdc10-C4 strains. These results suggest that DSC1Sp/MBF represses, as well as activates, MCB gene expression during the cell cycle.

Five new elements of the mitotic control in the fission yeast Schizosaccharomyces pombe were isolated from gene libraries as multicopy suppressors of the conditional lethal phenotype of win1-1 wee1ts cdc25ts triple mutant strains. These genes were designated wis1(+)-wis5+ for win suppressing, and do not correspond to win1+ or any of the previously characterised mitotic control genes. None of the wis genes is capable of suppressing the cdc phenotype of cdc25ts strains, suggesting that their effect is not simply to reverse the effect of loss of cdc25 function. wis1+ has been previously reported to encode a putative serine/threonine protein kinase that acts as a dosage-dependent inducer of mitosis. wis4+ appears to be a specific suppressor of the win1-1 mutation. wis2+ and wis3+ are capable of suppressing a wide range of cdc phenotypes arising from the combination of various mutations with wee1ts and cdc25ts, suggesting that the wis2+ and wis3+ products may interact with elements central to the mitotic control.

Cdc37 is a molecular chaperone that interacts with a range of clients and co-chaperones, forming various high molecular mass complexes. Cdc37 sequence homology among species is low. High homology between yeast and metazoan proteins is restricted to the extreme N-terminal region, which is known to bind clients that are predominantly protein kinases. We show that despite the low homology, both Saccharomyces cerevisiae and human Cdc37 are able to substitute for the Schizosaccharomyces pombe protein in a strain deleted for the endogenous cdc37 gene. Expression of a construct consisting of only the N-terminal domain of S. pombe Cdc37, lacking the postulated heat-shock protein (Hsp) 90-binding and homodimerization domains, can also sustain cellular viability, indicating that Cdc37 dimerization and interactions with the cochaperone Hsp90 may not be essential for Cdc37 function in S. pombe. Biochemical investigations showed that a small proportion of total cellular Cdc37 occurs in a high molecular mass complex that also contains Hsp90. These data indicate that the N-terminal domain of Cdc37 carries out essential functions independently of the Hsp90-binding domain and dimerization of the chaperone itself.

We have isolated 150 benomyl resistant mutants of the fission yeast Schizosaccharomyces pombe. Seven of these mutants were found to be cold sensitive for mitosis. These mutants were the subject of physiological, cytological and genetical characterisation. Growth and division of the seven mutants were similar to the wild type strain at 35 °C. After shift from the permissive (35 °C) to the restrictive temperature (20 °C) the mutants became blocked in mitosis whilst cellular growth continued. Consequently, elongate cells were formed. Six of the seven benomyl resistant mutants became blocked in mitosis at 20 °C with a single aberrant nucleus. In every case the benomyl resistant and cold sensitive phenotype was due to a mutation in a single nuclear gene. These mutants were found to comprise a single genetic linkage group (ben4) and were unlinked to existing TBZ/MBC resistant mutants of S. pombe. Whilst no cross resistance was found in our mutants to TBZ, six of the seven mutants were super sensitive to the spindle poison CIPC. We believe that the phenotype exhibited by these mutants is consistent with a defective tubulin subunit.

The free tryptophan pool and the levels of two enzymes of tryptophan biosynthesis (anthranilate synthase and indoleglycerolphosphate synthase) have been determined in a wild type strain of Saccharomyces cerevisiae and in mutants with altered regulatory properties.

The major cell cycle control in the fission yeast Schizosaccharomyces pombe acts at entry to mitosis, and involves three previously identified genes cdc2, cdc25 and wee1. The presence of a weel mutation phenotypically suppresses cdc25 mutations. This paper describes the isolation and subsequent analysis of a strain in which the suppression is reversed by the presence of a new mutation, designated win1.1. The mutation causes a slight increase in cell size at division in most genetic backgrounds. However, when combined with a wee1 mutation and cdc25.22, the win 1.1 mutation interacts strongly to generate a novel phenotype: cells are phenotypically cdc during growth on minimal medium but cdc+ when cultured on complex medium. The win1 locus is unlinked to previously identified genes involved in mitosis.