Processing of telomeric DNA ends requires the passage of a replication fork

During telomere replication in yeast, chromosome ends acquire a long single -stranded extension of the strand making the 3' end. Previous work showed t hat these 3' tails are generated late in S-phase, when conventional replica tion is virtually complete. In addition, the extensions were also observed in cells that lacked telomerase. Therefore, a model was proposed that predi cted an activity that recessed the 5' ends at yeast telomeres after convent ional replication was complete. Here, we demonstrate that this processing a ctivity is dependent on the passage of a replication fork through yeast tel omeres. A non-replicating linear plasmid with telomeres at each end does no t acquire single-stranded extensions, while an identical construct containi ng an origin of replication does. Thus, the processing activity could be as sociated with the enzymes at the replication fork itself, or the passage of the fork through the telomeric sequences allows a transient access for the activity to the telomeres. We therefore propose that there is a mechanisti c link between the conventional replication machinery and telomere maintena nce.