CIS-ACTING DETERMINANTS AFFECTING CENTROMERE FUNCTION, SISTER-CHROMATID COHESION AND RECIPROCAL RECOMBINATION DURING MEIOSIS IN SACCHAROMYCES-CEREVISIAE

We have employed a system that utilizes homologous pairs of human DNA- derived yeast artificial chromosomes (YACs) as marker chromosomes to a ssess the specific role(s) of conserved centromere DNA elements (CDEI, CDEII and CDEIII) in meiotic chromosome disjunction fidelity. Thirtee n different centromere (CEN) mutations were tested for their effects o n meiotic centromere function. YACs containing a wild-type CENDNA sequ ence segregate with high fidelity in meiosis I (99% normal segregation ) and in meiosis II (96% normal segregation). YACs containing a 31-bp deletion mutation in centromere DNA element II (CDEII Delta 31) in eit her a heterocentric (mutant/wild type), homocentric (mutant/mutant) or monosomic (mutant/-) YAC pair configuration exhibited high levels (16 -28%) of precocious sister-chromatid segregation (PSS) and increased l evels (1 -6%) of nondisjunction meiosis I (NDI). YACs containing this mutation also exhibit high levels (21%) of meiosis II nondisjunction. Interestingly, significant alterations in homolog recombination freque ncy were observed in the exceptional PSS class of tetrads, suggesting unusual interactions between prematurely separated sister chromatids a nd their homologous nonsister chromatids. We also have assessed the me iotic segregation effects of rare gene conversion events occurring at sites located immediately adjacent to or distantly from the centromere region. Proximal gene conversion events were associated with extremel y high levels (60%) of meiosis I segregation errors (including both PS S and NDI), whereas distal events had no apparent effect. Taken togeth er, our results indicate a critical role for CDEII in meiosis and unde rscore the importance of maintaining sister-chromatid cohesion for pro per recombination in meiotic prophase and for proper disjunction in me iosis I.