Structural and temporal analysis of telomere replication in yeast

There is a temporal order to eukaryotic chromo-somal replication (for review, see Fangman and Brewer 1992). In higher eukaryotes, this temporal regulation appears to reflect, at least in part, differences in gene expression. Chromosomal regions that are expressed or potentially expressible tend to replicate early, and regions that are transcriptionally inert a e replicated relativ~ly late in S phase (Goldman et al. 1984). Sac-charomyces chromosomes also have a temporal order of replication. Although most yeast chromosomal origins are activated early in S phase, origins near telomeres are activated late in S phase (Ferguson et al. 1991; Wellinger et al. 1993b). As a consequence, chro-mosomal regions near most origins are replicated early in S phase, whereas DNA replicated from telomere-proximal origins is replicated late in S phase. In at least one case--the nd of the left arm of chromosome I I I--late replication of subtelomeric DNA results from a lack of active origins in telomere-adjacent sequences (Reynolds et al. 1989; Zhu et al. 1992). As in higher cells, the late-replicating regions near yeast telomeres are transcriptionally repressed. When genes are moved near yeast telomeres, their constitutive transcription is reversibly repressed, a phenomenon called telomere position effect (Gottschling et al. 1990). Telo-meres also exert a position effect on the time of replica-tion of nearby DNA. A replication origin used early in S phase when on a circular plasmid (Wellinger et al. 1993b) or in its normal location near the centro-mere of chromosome IV (Ferguson and Fangman 1992) is used late in S phase when positioned near a telomere (Wellinger et al. 1993b; B. Brewer, pers. comm.). The very ends of eukaryotic hromosomes are proba-bly replicated by a different mechanism from that used for the rest of the chromosome. Telomeres bear simple, repeated DNA sequences. For example, chromosomes in Saccharomyces cerevisiae end in approximately 200-400 bp of a heterogeneous sequence, C2_3A(CA)I_6/ (TG)I_6TG2 _ ~, hereafter eferred to as CI_3A/TGt_