THE ESSENTIAL SCHIZOSACCHAROMYCES-POMBE CDC23 DNA-REPLICATION GENE SHARES STRUCTURAL AND FUNCTIONAL HOMOLOGY WITH THE SACCHAROMYCES-CEREVISIAE DNA43 (MCM10) GENE

The fission yeast cdc23 gene is required for correct DNA replication: cdc23 mutants show reduced rates of DNA synthesis and become elongated after cell-cycle arrest. We have cloned the Schizosaccharomyces pombe cdc23 gene by complementation of the temperature-sensitive phenotype of cdc23-M36 and confirmed the identity of the gene by integrative map ping. Analysis of the DNA sequence reveals that cdc23 can encode a pro tein of 593 amino acids (M-r=67 kDa) with 22% overall identity and man y structural homologies with the product of the Saccharomyces cel cere visiae DNA43 (MCM10) gene which is required for correct initiation of DNA synthesis at chromosomal origins of replication. Construction of a cdc23 null allele has established that the cdc23 gene is essential fo r viability, with cdc23 deletion mutant spores germinating but undergo ing arrest with undivided nuclei in the first or second cell cycle. Th e S. pombe cdc23 gene on an expression plasmid is able to complement t he S. cerevisiae dna43-1 mutant. These structural and functional homol ogies between two distantly related species suggest that cdc23 and DNA 43 may represent genes for a conserved essential eukaryotic DNA replic ation function.