Early and late replicating DNA involved in the G1 to S transition in Allium cepa L meristematic cells.

The involvement of portions of the genome replicated at different times of the S period in the regulation of the G1 to S transition was analyzed in Allium cepa L meristem cells. For this, DNA bromosubstitution confined to discrete portions of a previous S period followed by anoxic UVA irradiation (300-400 nm light) was performed in synchronous cells. Sequences replicated in late S appeared to be involved in the positive regulation of the initiation of replication. Hence, cells were prevented from initiating replication if irradiated at mid G1 only when the DNA sequences replicated in the last third of the previous S period were bromosubstituted. Cycloheximide-induced inhibition of protein synthesis at late G1 also prevented the G1 to S transition. Sequences replicated in mid S appeared unrelated to any control of the initiation of replication. On the other hand, sequences replicated in the first third of the S period seemed to be involved in the negative regulation of the initiation of replication, since irradiation after previous bromosubstitution of early replicating DNA sequences advanced G1 cells into the next S phase and increased the proliferative fraction of the population. Finally, the simultaneous inactivation of DNA sequences involved in both positive and negative regulation of replication allowed the cells to enter into S.