MCM21 and MCM22, two novel genes of the yeast Saccharomyces cerevisiae are required 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. This decrease in stability was associated with an increase in the copy number of the minichromosome in cells carrying it. Small circular dicentric plasmids were maintained relatively stably and structurally intact in the mutants compared with the wild-type strain. In the latter, such plasmids were mitotically unstable and, upon recovery, showed frequent rearrangements of their DNA. A centromere offered less obstruction to transcription in mutant cells than in the wild type, showing that both these mutants had a more relaxed kinetochore assembly. The mutant strains showed elevated rates of chromosome loss but not those of recombination. Both the mutations caused the cells to display a higher sensitivity towards the anti-mitotic drug benomyl. All these observations suggest that MCM21 and MCM22 are important for chromosome segregation with a potential role in kinetochore function. These genes are non-essential, 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 cells with a single nucleus at the neck. Furthermore, the DNA content of these cells showed a shift towards 2N, suggesting a temporary pause of cells in G2 or in an early phase of mitosis. The mcm21 and mcm22 mutations do not show synthetic lethality or any further enhancement of growth defects, implying that they could be carrying out non-overlapping functions in chromosome segregation.