FISSION YEAST GENES WHICH DISRUPT MITOTIC CHROMOSOME SEGREGATION WHENOVEREXPRESSED

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, Overexp ression of cDNAs derived from four genes, two known (nda3(+) and ubc4( +), encoding beta-tubulin and a ubiquitin conjugating enzyme, respecti vely) and two unknown, named mlo2(+) and mlo3(+) (missegregation & let hal when over expressed) caused phenotypes consistent with a failure t o segregate chromosomes, Full overexpression of all four cDNAs was let hal, Cells overexpressing nda3(+) and ubc4(+) cDNAs arrested with cond ensed unsegregated chromosomes and cells overexpressing mlo2(+) displa yed an asymmetric distribution of nuclear chromatin, Sublethal levels of overexpression of nda3(+), ubc4(+) and mlo2(+) cDNAs caused elevate d rates of minichromosome loss, A third cDNA mlo3(+), displayed no inc rease in the frequency of minichromosome loss at sublethal levels of o verexpression but full overexpression caused a complete failure to seg regate chromosomes, Our results confirm the assumption that beta-tubul in overexpression is lethal in S.pombe, implicate ubc4(+) in the contr ol of metaphase-anaphase transition in fission yeast and finally ident ify two new genes, mlo2(+) and mlo3(+), likely to play an important ro le for chromosome transmission fidelity in mitosis.