FAITHFUL CHROMOSOME TRANSMISSION REQUIRES SPT4P, A PUTATIVE REGULATOROF CHROMATIN STRUCTURE IN SACCHAROMYCES-CEREVISIAE

A chromosome transmission fidelity (ctf) mutant, s138, of Saccharomyce s cerevisiae was identified by its centromere (CEN) transcriptional re adthrough phenotype, suggesting perturbed kinetochore integrity in viv o. The gene complementing the s138 mutation was found to be identical to the S. cerevisiae SPT4 gene. The s138 mutation is a missense mutati on in the second of four conserved cysteine residues positioned simila rly to those of zinc finger proteins, and we henceforth refer to the m utation as spt4-138. Both spt4-138 and spt4 Delta strains missegregate a chromosome fragment at the permissive temperature, are temperature sensitive for growth at 37 degrees C, and upon a shift to the nonpermi ssive temperature show an accumulation of large budded cells, each wit h a nucleus. Previous studies suggest that Spt4p functions in a comple x with Spt5D and Spt6p, and we determined that spt6-140 also causes mi ssegregation of a chromosome fragment, Double mutants carrying spt4 De lta 2:: HIS3 and kinetochore mutation ndc 10-42 or cft13-30 show a syn thetic conditional phenotype. Both spt4-138 and spt4 Delta strains exh ibit synergistic chromosome instability in combination,vith CEN DNA mu tations and show in vitro defects in microtubule binding to minichromo somes, These results indicate that Spt4p plays a role in chromosome se gregation. The results of in vivo genetic interactions with mutations in kinetochore proteins and CEN DNA and of in vitro biochemical assays suggest that Spt4p is important for kinetochore function.