We have isolated STNl, an essential Saccbaromyces cerevisiae gene, as a suppressor of the cdcl3-l mutation. A synthetic lethal interaction between a temperature-sensitive mutant allele of STNl, stnl-13, and cdcl3-l was observed. Stnl and Cdcl3 proteins displayed a physical interaction by two-hybrid analysis. As shown previously for cdcl3-l, stnl-13 cells at the restrictive temperature accumulate single-stranded DNA in subtelomeric regions of the chromosomes, but to a lesser extent than cdcl3-l cells. In addition, both Cdcl3 and Stnl were found to be involved in the regulation of telomere length, mutations in STNl or CDC13 conferring an increase in telomere size. Loss of Stnl function activated the RAD9 and MEC3 G2/M checkpoints, therefore confirming that DNA damage is generated. We propose that Stnl functions in telomere metabolism during late S phase in cooperation with Cdcl3.

In Saccharomyces cerevisiae, Cdc13 has been proposed to mediate telomerase recruitment at telomere ends. Stn1, which associates with Cdc13 by the two-hybrid interaction, has been implicated in telomere maintenance. Ten1, a previously uncharacterized protein, was found to associate physically with both Stn1 and Cdc13. A binding defect between Stn1-13 and Ten1 was responsible for the long telomere phenotype of stn1-13 mutant cells. Moreover, rescue of the cdc13-1 mutation by STN1 was much improved when TEN1 was simultaneously overexpressed. Several ten1 mutations were found to confer telomerase-dependent telomere lengthening. Other, temperature-sensitive, mutants of TEN1 arrested at G 2 /M via activation of the Rad9-dependent DNA damage checkpoint. These ten1 mutant cells were found to accumulate singlestranded DNA in telomeric regions of the chromosomes. We propose that Ten1 is required to regulate telomere length, as well as to prevent lethal damage to telomeric DNA.

We have studied the patterns of expression of four B-type cyclins (Clbs), Cibl, Clb2, Clb3, and 5, 6, 22, 35, 36, 41, 45, and 50]). This 34-kDa protein kinase, p34cdc2, is present at a constant level during the cell cycle but exhibits a periodic activity. In all systems studied so far, the activity associated with p34cdc2 rises to a peak during M phase and falls to a low level during interphase (la, 7, 15, 28, 40). Furthermore, this activation of p34C c2 at the G2/M-phase transition has been shown to be necessary for induction of mitosis (42). A decrease in p34CdC2 -associated kinase activity allows exit from mitosis and entry into the next interphase (17,34,43). Genetic analysis in the yeasts Schizosaccharomyces pombe and S. cerevisiae has revealed a second role for p34 in the transition from G1 to S phase. Specifically, activation of p34 kinase is required for the passage through the primary cell cycle regulatory checkpoint known as START (20,46,57). p34 kinase activity is regulated by a class of proteins known as cyclins. First identified in eggs and embryos of marine invertebrates (13), cyclins have subsequently been found to be ubiquitous in eukaryotes ranging from yeasts to humans (2,17,19,21,38,39,52,53,66,68,70,71,76; for reviews, see references 22 and 32). Cyclins are positive regulatory subunits of p34 kinase, and the periodic synthesis and degradation of most cyclins during the cell cycle accounts, to a large degree, for the periodic activation of the kinase. In most systems that have been studied, cyclins of the A and B types have been shown to accumulate up until the G2/M-phase transition and are thought to be responsible for mitotic induction (29,30,42,48). In Xenopus spp. and clams, these same cyclins have been shown to be involved in regulation of meiotic metaphase and the subsequent meiotic * Corresponding author.

Cdc13 performs an essential function in telomere end protection in budding yeast. Here, we analyze the consequences on telomere dynamics of cdc13-induced telomeric DNA damage in proliferating cells. Checkpoint-de®cient cdc13-1 cells accumulated DNA damage and eventually senesced. However, these telomerase-pro®cient cells could survive by using homologous recombination but, contrary to telomerasede®cient cells, did so without prior telomere shortening. Strikingly, homologous recombination in cdc13-1 mec3, as well as in telomerase-de®cient cdc13-1 cells, which were Rad52-and Rad50-dependent but Rad51-independent, exclusively ampli®ed the TG 1±3 repeats. This argues that not only short telomeres are substrates for type II recombination. The Cdc13-1 mutant protein harbored a defect in its association with Stn1 and Ten1 but also an additional, unknown, defect that could not be cured by expressing a Cdc13-1± Ten1±Stn1 fusion. We propose that Cdc13 prevents telomere uncapping and inhibits recombination between telomeric sequences through a pathway distinct from and complementary to that used by telomerase.