A YEAST ORIGIN OF REPLICATION IS ACTIVATED LATE IN S-PHASE

The mechanism that causes large regions of eukaryotic chromosomes to remain unreplicated until late in S phase is not understood. We have found that 67 kb of telomere-adjacent DNA at the right end of chromosome V in S. cerevisiae is replicated late in S phase. An ARS element in this region, ARS501, was shown by two-dimensional gel analysis to be an active origin of replication. Kinetic analyses indicate that the rate of replication fork movement within this late region is similar to that in early replicating regions. Therefore, the delayed replication of the region is a consequence of late origin activation. The results also support the idea that the pattern of interspersed early and late replication along the chromosomes of higher eukaryotes is a consequence of the temporal regulation of origin activation.