MUTATIONS SYNTHETICALLY LETHAL WITH CEP1 TARGET SACCHAROMYCES-CEREVISIAE KINETOCHORE COMPONENTS

CP1 (encoded by CEP1) is a Saccharomyces cerevisiae chromatin protein that binds a DNA element conserved in centromeres and in the 5'-flanki ng DNA of methionine biosynthetic (MET) genes. Strains lacking CPI are defective in chromosome segregation and MET gene transcription, leadi ng to the hypothesis that CP1 plays a general role in assembling highe r order chromatin structures at genomic sites where it is bound. A scr een for mutations synthetically lethal with a cep1 null allele yielded five recessive csl (cep1 synthetic lethal) mutations, each defining a unique complementation group. Four of the five mutations synergistica lly increased the loss rate of marker chromosomes carrying a centromer e lacking the CPI binding site, suggesting that the cep1 synthetic let hality was due to chromosome segregation defects. Three of these four CSL genes were subsequently found to be known or imputed kinetochore g enes: CEP3 NDC10, and CSE4. The fourth, CSL4, corresponded to ORF YNL2 32w on chromosome XIV, and was found to be essential. A human cDNA was identified that encoded a protein homologous to Csl4 and that complem ented the csl4-1 mutation. The results are consistent with the view th at the major cellular role of CP1 is to safeguard the biochemical inte grity of the kinetochore.