Fission yeast genes which disrupt mitotic chromosome segregation when overexpressed.

An interference assay has been devised in Schizosaccharomyces pombe to rapidly identify and clone genes involved in chromosome segregation. Random S.pombe cDNAs were overexpressed from an inducible promoter in a strain carrying an additional, non-essential minichromosome. Overexpression of cDNAs derived from four genes, two known (nda3+and ubc4+, encoding beta-tubulin and a ubiquitin conjugating enzyme, respectively) and two unknown, named mlo2+ and mlo3+ (missegregation & lethal when over expressed) caused phenotypes consistent with a failure to segregate chromosomes. Full overexpression of all four cDNAs was lethal. Cells overexpressing nda3+ and ubc4+ cDNAs arrested with condensed unsegregated chromosomes and cells overexpressing mlo2+ displayed an asymmetric distribution of nuclear chromatin. Sublethal levels of overexpression of nda3+, ubc4+ and mlo2+ cDNAs caused elevated rates of minichromosome loss. A third cDNA mlo3+, displayed no increase in the frequency of minichromosome loss at sublethal levels of overexpression but full overexpression caused a complete failure to segregate chromosomes. Our results confirm the assumption that beta-tubulin overexpression is lethal in S.pombe, implicate ubc4+ in the control of metaphase-anaphase transition in fission yeast and finally identify two new genes, mlo2+and mlo3+, likely to play an important role for chromosome transmission fidelity in mitosis.