Abstract
Telomeres of the fission yeast, Schizosaccharomyces pombe, are known to replicate in late S phase, but the reasons for this late replication are not fully understood. We have identified two closely-spaced DNA replication origins, 5.5 to 8 kb upstream from the telomere itself. These are the most telomere-proximal of all the replication origins in the fission yeast genome. When located by themselves in circular plasmids, these origins fired in early S phase, but if flanking sequences closer to the telomere were included in the circular plasmid, then replication was restrained to late S phase – except in cells lacking the replication-checkpoint kinase, Cds1. We conclude that checkpoint-dependent late replication of telomere-associated sequences is dependent on nearby cis-acting sequences, not on proximity to the physical end of a linear chromosome.
Highlights
The genome of the fission yeast, Schizosaccharomyces pombe, is organized into three chromosomes with a total of six proteinDNA telomere structures
Attempt to localize the Telomeric HindIII Fragment (THF) sequences that regulate replication timing we examined the effects of deleting various sequences from the THF, while leaving R1 intact
We found that all of the origin activity was confined to region 1, which in its chromosomal context is 6–8 kb from the simple-sequence repeats at chromosome ends (Figure 2)
Summary
The genome of the fission yeast, Schizosaccharomyces pombe, is organized into three chromosomes with a total of six proteinDNA telomere structures. Chromosome 3 contains arrays of ribosomal DNA (rDNA) repeats near both ends In some strains, these rDNA repeats directly abut the simple-sequence repeats[1], while in other strains there are TAS between the rDNA and the simplesequence repeats at one or both chromosome 3 ends[1,4,5]. The complete TAS array consists of approximately 50 kb, forming large inverted repeats at the ends of chromosomes 1 and 2 (Figure 1A). Most of this sequence is “unique” (except for the fact that each cell contains 4–6 copies of TAS), but there are clusters of direct repeats within the first 3 kb and 4 to 6 kb away from the telomere (Figure 1B, C). The right end of chromosome 2 provides an excellent model for the structure of the other chromosome ends (Figure 1A)
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