THE MCM17 MUTATION OF YEAST SHOWS A SIZE-DEPENDENT SEGREGATIONAL DEFECT OF A MINICHROMOSOME

Mini-chromosome-maintenance (mcm) mutants were described earlier as ye ast mutants which could not stably maintain mini-chromosomes. Out of t hese, the ARS-specific class has been more extensively studied and is found to lose chromosomes and mini-chromosomes due to a defect in the initiation of DNA replication at yeast ARSs. In the present study we h ave identified a number of mcm mutants which show size-dependent loss of minichromosomes. When the size of the mini-chromosome was increased , from about 15 kb to about 60 kb, there was a dramatic increase in it s mitotic stability in these mutants, but not in the ARS-specific clas s of mutants. One mutant, mcm17, belonging to the size-dependent class was further characterized. In this mutant, cells carried mini-chromos omes in significantly elevated copy numbers, suggesting a defect in se gregation. This defect was largely suppressed in the 60-kb mini-chromo some. A non-centromeric plasmid, the TRP1ARS1 circle, was not affected in its maintenance. This mutant also displayed enhanced chromosome-II I loss during mitosis over the wild-type strain, without elevating mit otic recombination. Cloning and sequencing of MCM17 has shown it to be the same as CHL4, a gene required for chromosome stability. This gene is nonessential for growth, as its disruption or deletion from the ch romosome did not affect the growth-rate of cells at 23 degrees C or 37 degrees C, This work suggests that centromere-directed segregation of a chromosome in yeast is strongly influenced by its length.