Somatic pairing of homologs in budding yeast: existence and modulation

FISH analysis of well-spread chromosomes reveals that homologs are paired in vegetatively growing budding yeast diploid cells, via multiple interstitial interactions, and independent of recA homologs and mating type heterozygosity. 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 pairing is qualitatively analogous to premeiotic and early meiotic pairing. S-phase pairing disruption occurs by a complex intranuclear program involving regional, nucleus-wide, and temporal determinants. Pairing is also disrupted in two G(2)-arrest conditions (cdc13ts and nocodazole). Together these findings suggest that cell cycle signals may provoke pairing disruption by modulating 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 functional considerations involving homolog/sister discrimination.