[Stable maintenance of dicentric mini-chromosomes in CHL4 mutants in yeast]

Earlier we have identified the chl4-1 mutation in a screen for yeast mutants with increased loss of chromosome III and circular artificial minichromosome in mitosis. Mutation in the CHL4 gene leads to a 50-100-fold promotion in the rate of chromosome loss per cell division compared to the isogenic wild type strain. Detailed analysis of behaviour of the circular minichromosome marked by the CUP1 gene has shown that minichromosome nondisjunction (2:0 segregation) leading to an increase in the copy number of minichromosome in part of a cell population is the main reason of minichromosome instability in the mutant. The unique peculiarity of chl4-1 mutation is the ability of the strains carrying this mutation to stably maintain circular dicentric minichromosomes without any rearrangement during many generations. (In the wild type strains dicentric minichromosomes are extremely unstable. As a consequence of that there is a strong selection for cells harboring monocentric derivatives in a population of cells derived from a cell containing a dicentric plasmid). Introduction of the second centromere into one of the natural chromosomes (chromosomes II or III) in the chl4-1 mutant leads to the same dramatic consequences as that in the wild type strain (mitotic lag of cells harboring dicentric chromosomes and, as a result of that, selective pressure for cells harboring monocentric derivatives of dicentric chromosome). A genomic clone of CHL4 was isolated by complementation of the chl4-1 mutation. Nucleotide sequence analysis of CHL4 revealed a 1.4-kb open reading frame with a predicted 53-kDa protein sequence. Analyzing the sequence of the CHL4 protein we have found a region meeting the necessary requirements for the helix-turn-helix (HTH) structure. This region of the CHL4 protein has about 40% homology with the repressor of tryptophane operon (TrpR) of E. coli. A strain containing a null allele of CHL4 was viable under standard growth conditions, but had temperature-sensitive phenotype (conditional lethality at 34 degrees C). We suggest that the CHL4 gene product is one of the components of the segregation cell machinery.