A probe into nuclear events during the cell cycle of Saccharomyces cerevisiae: studies of folded chromosomes in cdc mutants which arrest in G1.

The sedimentation behavior of folded chromosomes from cell-division-cycle (cdc) mutants which arrest in G1 was examined. At the restrictive temperature the folded genome of cdc 7, which arrests after spindle pole body (SPB) separation and spindle formation, cosediments with a standard g1 structure, indicating that by the cdc 7 step the g1 form of the folded genome has been assembled. In the mutant, cdc 4, which arrests before SPB separation but after SPB duplication, a standard g1 structure is not formed. cdc 4 cells, however, are able to enter G0 at the restrictive temperature, and the corresponding go structure is stable. These results indicate that the cdc 4 gene product may be involved in the development of folded genome conformation which leads to the g1 structure. Since the cdc 4 gene product is required for SPB separation, the g1 structure may be defined by an association between chromosomes and spindle components. The folded chromosomes of the "start" mutants cdc 25 and cdc 28 are unstable at the restrictive temperature. In contrast to cdc 4, neither cdc 25 nor cdc 28 are able to enter the G0 stage in a normal manner, i.e., the g0 structure is unstable at the restrictive temperature. The inference is that both the cdc 25 and cdc 28 gene products are required for the functional integrity of the folded genome at both a stage early in G1 and in the pathway to G0.