Fission yeast cdc24(+) encodes a novel replication factor required for chromosome integrity.

A mutation within the Schizosaccharomyces pombe cdc24 1 gene was identified previously in a screen for cell division cycle mutants and the cdc24 1 gene was determined to be essential for S phase in this yeast. We have isolated the cdc24 1 gene by complementation of a new temperature-sensitive allele of the gene, cdc24-G1. The DNA sequence predicts the presence of an open reading frame punctuated by six introns which encodes a pioneer protein of 58 kD. A cdc24 null mutant was generated by homologous recombina- tion. Haploid cells lacking cdc24 1 are inviable, indicating that cdc24 1 is an essential gene. The transcript of cdc24 1 is present at constant levels throughout the cell cycle. Cells lacking cdc24 1 function show a checkpoint-dependent arrest with a 2N DNA content, indicating a block late in S phase. Arrest is accompa- nied by a rapid loss of viability and chromosome breakage. An S. pombe homolog of the replicative DNA helicase DNA2 of S. cerevisiae suppresses cdc24. These results suggest that Cdc24p plays a role in the progression of normal DNA replication and is required to maintain genomic integrity.