MCM21 and MCM22, two novel genes of the yeast Saccharomyces cerevisiae arerequired for chromosome transmission

The minichromosome maintenance genes, MCM21 and MCM22, have been cloned and are shown to code for the ORFs YDR318W and YJR135C respectively. Mutations in these genes caused a decrease in the stability of the minichromosome. T his decrease in stability was associated with an increase in the copy numbe r of the minichromosome in cells carrying it. Small circular dicentric plas mids were maintained relatively stably and structurally intact in the mutan ts compared with the wild-type strain. In the latter, such plasmids were mi totically unstable and, upon recovery, showed frequent rearrangements of th eir DNA. A centromere offered less obstruction to transcription in mutant c ells than in the wild type, showing that both these mutants had a more rela xed kinetochore assembly. The mutant strains showed elevated rates of chrom osome loss but not those of recombination. Both the mutations caused the ce lls to display a higher sensitivity towards the anti-mitotic drug benomyl. All these observations suggest that MCM21 and MCM22 are important for chrom osome segregation with a potential role in kinetochore function. These gene s are nonessential, as their deletions from chromosomes did not cause loss of cell viability. However, exponentially growing mutant cells carrying the deletion of the MCM21 gene had a significant population of large-budded ce lls with a single nucleus at the neck. Furthermore, the DNA content of thes e cells showed a shift towards 2N, suggesting a temporary pause of cells in GZ or in an early phase of mitosis. The mcm21 and mcm22 mutations do not s how synthetic lethality or any further enhancement of growth defects, imply ing that they could be carrying out non-overlapping functions in chromosome segregation.