hsk1+, a Schizosaccharomyces pombe gene related to Saccharomyces cerevisiae CDC7, is required for chromosomal replication.

Degenerate oligonucleotide-directed polymerase chain reaction was conducted to clone a possible Schizosaccharomyces pombe homologue [hsk1 for a putative homologue of CDC7 (seven) kinase 1] of Saccharomyces cerevisiae Cdc7 kinase. The cloned cDNA for hsk1+ contains an open reading frame consisting of 507 amino acids with predicted mol. wt of 58,370 that possesses overall amino acid identity of 46% (65% including similar residues) to CDC7. In addition to conserved domains for serine-threonine kinases, the predicted primary structure of Hsk1 contains three 'kinase insert' sequences characteristic to Cdc7 at the positions identical to those of Cdc7. Whereas the length and sequences of the kinase inserts are diverged between the two yeast species, 58% identity (76% including similar residues) is detected within the kinase conserved domains. The hsk1+ gene, which is present as a single copy on the S.pombe chromosome, contains two introns within the coding frame. Disruption of the hsk1+ gene by insertion of the ura4+ gene is lethal to growth. Analysis of the DNA content of germinating spores that contain hsk1 null alleles indicates that DNA replication is inhibited in the mutant. The morphology of these mutant spores after germination indicates abnormal nuclear division in some population of germinating spores, suggesting either that Hsk1 may be required for inhibition of mitosis until completion of S phase or that it may also be involved in proper execution of mitosis. Our results suggest that hsk1+ is a strong candidate for the functional fission yeast homologue of budding yeast CDC7 and that a mechanism through which initiation of chromosomal replication is regulated may be conserved between the two yeast species.