THE RHC21 GENE OF BUDDING YEAST, A HOMOLOG OF THE FISSION YEAST RAD21(+) GENE, IS ESSENTIAL FOR CHROMOSOME SEGREGATION

The Saccharomyces cerevisiae gene RHC21 is a homologue of the fission yeast rad21(+) gene, which affects the sensitivity of cells to gamma-i rradiation and is essential for cell growth in S. pombe. Disruption of the RHC21 gene showed that it is also essential in S. cerevisiae. To examine its function in cell growth further, we have isolated temperat ure-sensitive mutants for the RHC21 gene and characterized one of them , termed rhc21-sk16. When this mutant was incubated at 36 degrees C, t he percentage of large-budded cells was increased. Most of the large-b udded cells had aberrant nuclear structures, such as unequally extende d nuclear DNA with incompletely elongated spindles across the mother-d aughter neck or only in a mother cell. Furthermore, a circular minichr omosome is more unstable in the mutant than in the wild-type, even at 25 degrees C. Flow cytometry showed that the bulk of DNA replication t akes place normally at the restrictive temperature in the mutant. Thes e results indicated that the RHC21 gene is required for proper segrega tion of the chromosomes. In addition, we found that the mutant is sens itive not only to UV radiation and gamma-rays but also to the antimicr otubule agent nocodazole at 25 degrees C. This suggests that the RHC21 gene is involved in the microtubule function. We discuss how the RHC2 1 gene product may be involved in chromosome segregation and microtubu le function.