FISSION YEAST GENES NDA1(-PHASE BLOCK, CHROMATIN ALTERATION AND CA2+ SUPPRESSION, ARE MEMBERS OF THE CDC46() AND NDA4(+), MUTATIONS OF WHICH LEAD TO S)MCM2 FAMILY/

Fission yeast cold-sensitive mutants nda1-376 and nda4-108 display a c ell cycle block phenotype at the restrictive temperature (cell elongat ion with the single nucleus) accompanied by an alteration in the nucle ar chromatin region. DNA content analysis shows that the onset of DNA synthesis is blocked or greatly delayed in both mutant cells, the bloc k being reversible in nda4-108. Upon release to the permissive tempera ture, nda4-108 cells resumed replicating DNA, followed by mitosis and cytokinesis. The nda4 phenotype was partly rescued by the addition of Ca2+ to the medium; Ca2+ plays a positive role in the nda4(+) function . The predicted protein sequences of nda1(+) and nda4(+) isolated by c omplementation are similar to each other and also, respectively, to th ose of the budding yeast, MCM2 and CDC46, both of which are members of the gene family required for the initiation of DNA replication. The c entral domains of these proteins are conserved, whereas the NH2- and C OOH- domains are distinct. Results of the disruption of the nda1(+) an d nda4(+) genes demonstrates that they are essential for viability.