Abstract. The fission yeast Schizosaccharomyces pombe divides by medial fission and, like many higher eukaryotic cells, requires the function of an F-actin contractile ring for cytokinesis. In S. pombe, a class of cdc-mutants defective for cytokinesis, but not for DNA replication, mitosis, or septum synthesis, have been identified. In this paper, we present the characterization of one of these mutants, cdc3-124. Temperature shift experiments reveal that mutants in cdc3 are incapable of forming an F-actin contractile ring. We have molecularly cloned cdc3 and used the cdc3 + genomic DNA to create a strain carrying a cdc3 null mutation by homologous recombination in vivo. Cells bearing a cdc3-null allele are inviable. They arrest the cell cycle at cytokinesis without forming a contractile ring, DNA sequence analysis of the cdc3 + gene reveals that it encodes profilin, an actin-monomer-binding protein. In light of recent studies with profilins, we propose that Cdc3-profilin plays an essential role in cytokinesis by catalyzing the formation of the F-actin contractile ring. Consistent with this proposal are our observations that Cdc3-profilin localizes to the medial region of the cell where the F-actin contractile ring forms, and that it is essential for F-actin ring formation. Cells overproducing Cdc3-profilin become elongated, dumbbell shaped, and arrest at cytokinesis without any detectable F-actin staining. This effect of Cdc3-profilin overproduction is relieved by introduction of a multicopy plasmid carrying the actin encoding gene, act1 +. We attribute these effects to potential sequesteration of actin monomers by profilin, when present in excess.T I~E three major landmark events in the eukaryotic cell cycle are (a) replication of the genetic material that occurs in S phase; (b) partitioning of the replicated DNA to the daughter nuclei during M phase; and (c) division of the cell itself to produce two daughter cells as a result of cytokinesis. The last 10 yr have seen tremendous advances in our understanding of the molecular nature of processes occurring during the S and M phases and the controis that regulate entry into S and M phases. By contrast, the mechanisms and regulations that govern the third major landmark event, cytokinesis, remain only poorly understood.Much of the progress in our understanding of the controls that regulate S and M phases stem from genetic studies in the fission yeast Schizosaccharomyces pombe. Several lines of evidence suggest that this yeast should also be an ideal system with which to study cytokinesis. First, S. pombe cells divide by medial fission (Nurse, 1985) and, like many higher eukaryotic cells, produce equally sized daughter cells after cytokinesis. Second, cytokinesis in fission yeast, akin to

The Schizosaccharomyces pombe cdc5+ gene was identified in the first screen for cell division cycle mutants in this yeast. The cdc5+ gene was reported to be required for nuclear division but because of its modest elongation and leaky nature at the non‐permissive temperature, it was not investigated further. Here, we report the characterization of the single allele of this gene, cdc5‐120, in more detail. The mutant arrests with a 2N DNA content and a single interphase nucleus. Further genetic analyses suggest that cdc5+ gene function is essential in the G2 phase of the cell cycle. We have cloned and sequenced the cdc5+ gene. The deduced protein sequence predicts that Cdc5 is an 87 kDa protein and contains a region sharing significant homology with the DNA binding domain of the Myb family of transcription factors. Deletion mapping of the cdc5+ gene has shown that the N‐terminal 232 amino acids of the protein, which contain the Myb‐related region, are sufficient to complement the cdc5ts strain. A cdc5 null mutant was generated by homologous recombination. Haploid cells lacking cdc5+ are inviable, indicating that cdc5+ is an essential gene. A fusion protein consisting of bacterial glutathione S‐transferase joined in‐frame to the N‐terminal 127 amino acids of the Cdc5 protein is able to bind to DNA cellulose at low salt concentrations. This evidence suggests that cdc5+ might encode a transcription factor whose activity is required for cell cycle progression and growth during G2.