The mcm17 mutation of yeast shows a size-dependent segregational defect of a mini-chromosome.
Mini-chromosome-maintenance (mcm) mutants were described earlier as yeast mutants which could not stably maintain mini-chromosomes. Out of these, 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 have identified a number of mcm mutants which show size-dependent loss of mini-chromosomes. When the size of the mini-chromosome was increased, from about 15 kb to about 60 kb, there was a dramatic increase in its mitotic stability in these mutants, but not in the ARS-specific class of mutants. One mutant, mcm17, belonging to the size-dependent class was further characterized. In this mutant, cells carried mini-chromosomes in significantly elevated copy numbers, suggesting a defect in segregation. This defect was largely suppressed in the 60-kb mini-chromosome. A non-centromeric plasmid, the TRP1ARS1 circle, was not affected in its maintenance. This mutant also displayed enhanced chromosome-III loss during mitosis over the wild-type strain, without elevating mitotic recombination. Cloning and sequencing of MCM17 has shown it to be the same as CHL4, a gene required for chromosome stability. This gene is non-essential for growth, as its disruption or deletion from the chromosome 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.
Roy
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N
Department of Biochemistry, Bose Institute, Calcutta, India.
Poddar
A
A
Lohia
A
A
Sinha
P
P
D018797:Cell Cycle Proteins; D002872:Chromosome Deletion; D015825:Chromosomes, Fungal; D003001:Cloning, Molecular; D004261:DNA Replication; D005656:Fungal Proteins / Q000235:genetics*; D005800:Genes, Fungal; D009154:Mutation; D015183:Restriction Mapping; D029701:Saccharomyces cerevisiae Proteins
C081874:CHL4 protein, S cerevisiae; D018797:Cell Cycle Proteins; D005656:Fungal Proteins; D029701:Saccharomyces cerevisiae Proteins