Feedback control of telomere length D Shore and A Bianchi have a role in mediating repression of telomerase activity, as partial loss-of-function alleles of all three proteins cause telomere elongation. These ﬁndings lead the Lundblad and Charbonneau laboratories to propose models in which CST regulates telomere length by limiting telomerase access to the ends (Grandin et al, 2000; Chandra et al, 2001). The Lundblad laboratory, in particular, proposed that Cdc13 ﬁrst acts posi- tively in the telomerase pathway (by recruiting Est1) and then switches to an inhibitory role through interaction with Stn1 (Chandra et al, 2001). The repressive effect of CST appears to operate through an interaction between the C-terminal do- main of Stn1 and the Cdc13 protein (Chandra et al, 2001; Puglisi et al, 2008). Thus, even though the Cdc13–Est1 interaction promotes telomere elongation, CST as a whole can inhibit telomere elongation. The current model is that CST might do so by modulating the lagging-strand replication machinery responsible for synthesis of the CA-strand. Indeed, Cdc13 also interacts with the Pol1 (catalytic) subunit of DNA polymerase a/primase (Qi and Zakian, 2000), whereas Stn1 displays physical and functional association with the Pol1 regulatory subunit Pol12 (Grossi et al, 2004; Petreaca et al, 2007; Puglisi et al, 2008; Gasparyan et al, 2009). As point mutations in both Pol1 and Pol12 affecting their interaction with Cdc13 and Stn1, respectively, cause telomere elongation (Qi and Zakian, 2000; Grossi et al, 2004; Puglisi et al, 2008), the CST–DNA polymerase a/primase interaction is presumed to inhibit at least one step in the telomerase pathway, possibly by modulating the length of the TG overhang avail- able for Cdc13–Est1–telomerase action. This idea is consis- tent with the view, ﬁrst proposed in ciliates, that synthesis of the CA-strand by lagging-strand replication is inhibitory for TG-strand synthesis (Fan and Price, 1997). Very recently it has been proposed that the switch in Cdc13-binding partners from Est1 to Stn1 is modulated by a speciﬁc Cdk1-dependent phosphorylation event on the telomerase RD of Cdc13 (Li et al, 2009). In addition, an elegant biochemical study has just uncovered a role, at least in vitro, for the Hsp82 chaperone in facilitating a transition between Cdc13/telomer- ase extendable and CST non-extendable complexes (DeZwaan et al, 2009). This work points to a role for the Cdc13 C-terminus in repressing telomerase through assembly of the CST complex, in a manner not solely dependent on CA- strand synthesis, and in addition identiﬁes a function for the Cdc13 N-terminus in activating telomerase action indepen- dently of a recruitment function, in agreement with earlier genetic studies (Meier et al, 2001). Putting the pieces together: how does telomere tract length modulate telomerase action? The constellation of data outlined above supports the follow- ing general working model of telomerase-dependent telomere elongation (Figure 2). First, replication fork passage sets the stage for a DNA end-processing reaction that leads to the generation (or extension) of a 30 TG-rich single-stranded overhang on both leading- and lagging-strand ends. The Tel1 kinase and the MRX complex, which act in a common telomerase pathway for telomere maintenance (Ritchie and Petes, 2000), might promote this event, possibly through the recruitment and/or activation of multiple exonucleolytic activities. Genetic epistasis data suggest that some aspect of MRX/Tel1 function is directly inhibited in cis by the action of Rap1/Rif complexes bound to the duplex portion of the TG repeat tracts (Craven and Petes, 1999; Ray and Runge, 1999; Chan et al, 2001). Subsequent to end processing, the newly generated overhang promotes Cdc13 binding. (Whether this is a direct consequence of increased single-stranded binding sites for Cdc13 is still not clear (Tsukamoto et al, 2001).) Increased Cdc13 binding and/or a modiﬁcation of telomere-bound Cdc13 (see below), in turn leads to telomerase holoenzyme recruitment, at least in part through the Cdc13–Est1 interaction, and subsequent TG-strand extension. Finally, a CST interaction with DNA polymerase RIF1 RIF2 RAP1 MRX-Tel1 loading/resection MRX Exo Tel1 Cdc13 binding, phosphorylation by Tel1 Ten1 P Cdc13 Stn1 CST-mediated telomerase recruitment/activation Est1 Telomerase Est2 TLC1 Pola-primase binds CST, inhibits telomerase recruitment Pol1 P Primase Pol12 Figure 2 Proposed steps in telomere replication in the budding yeast S. cerevisiae. After passage of the DNA replication fork, the MRX (Mre11, Rad50, Xrs2) complex and the PI3K-related kinase Tel1 are recruited to the telomere ends. (Differences in leading and lagging-strand ends are omitted for simplicity). The MRX complex, Tel1 and Cdk1 (not indicated) control the exonucleolytic resection of the telomeric (CA-rich) 50 end through as yet uncharacterized exonucleases and mechanisms. The 30 single-strand overhang thus generated (or elongated) serves as a platform for binding of the CST (Cdc13–Stn1–Ten1) complex. Tel1 phosphorylates Cdc13 at multi- ple sites (Cdc13 is also phosphorylated by Cdk1). Phosphorylated Cdc13 recruits telomerase through an interaction with the Est1. Finally, an interaction between DNA polymerase a/primase complex and CST inhibits telomerase recruitment and promotes completion of lagging-strand synthesis (see text for details). 2314 The EMBO Journal VOL 28 | NO 16 | 2009 & 2009 European Molecular Biology Organization