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.