Somatic pairing of homologs in budding yeast: existence and modulation

FISH analysis of well-spread chromosomes reveals that homologs are paired i n vegetatively growing budding yeast diploid cells, via multiple interstiti al interactions, and independent of recA homologs and mating type heterozyg osity. Pairing is present during G(1) and G(2), and in cells arrested at G( 1) by mating pheromone, but is disrupted during S phase. Thus, somatic pair ing is qualitatively analogous to premeiotic and early meiotic pairing. S-p hase pairing disruption occurs by a complex intranuclear program involving regional, nucleus-wide, and temporal determinants. Pairing is also disrupte d in two G(2)-arrest conditions (cdc13ts and nocodazole). Together these fi ndings suggest that cell cycle signals may provoke pairing disruption by mo dulating underlying chromosome and/or chromatin structure. Whether the cell chooses to disrupt pairing contacts or not (e.g., S phase and G(2) arrest, but not G(1) arrest or normal G(1) or G(2)), could be dictated by function al considerations involving homolog/sister discrimination.