[The causes of instability of artificial mini-chromosomes in yeasts mutant for chl genes]

In mutants chl2, chl3, chl5, and chl6, which control mitotic chromosome transmission, the behaviour of the centromeric plasmids with various genes was analyzed. The main cause of chromosome instability in chl2, chl5, and chl6 is chromosome loss during cell division (1:0 segregation). The main cause of chromosome instability in chl3. is nondisjunction (2:0 segregation). According to this, the chl3 mutant, but not other chl's, cannot maintain the mini-chromosome with SUP11 gene. This gene causes cell death in high copy number. Chromosome nondisjunction in chl3 is also confirmed by the data on the mini-chromosome carrying CUP1 gene responsible for copper-resistance in yeast. The copper resistancy level in chl3 transformants is much higher than in chl5 or wild type transformants. Elevated copper resistance of chl3 transformants is caused by the transit accumulation of CUP1-carrying mini-chromosome in part of the cell population as a result of segregation mistakes upon cell divisions. Thus, the CHL3 gene is a new gene that controls the process of mitotic chromosome disjunction in yeast.