The finger subdomain of yeast telomerase cooperates with Pif1p to limit telomere elongation.

Telomere synthesis depends on telomerase, which contains an RNA subunit linked to a specialized reverse transcriptase subunit and several associated proteins. Here we report the characterization of four mutations in the yeast reverse transcriptase subunit Est2p that cause an overelongation of telomeres and an increase in the association of Est1p with telomeres during S phase. These 'up-mutations' are clustered in the finger subdomain of the reverse transcriptase. We show that the catalytic properties of the up-mutant telomerases are not improved in vitro. In vivo, the up-mutations neither bypass the activation step governed by Cdc13p nor do they uncouple telomerase from the Rap1p inhibition pathway. In the presence of the up-mutations, however, the ability of the Pif1p helicase to decrease telomere length and to inhibit the association of Est1p with telomeres is impaired. In addition, Pif1p associates in vivo with the telomerase RNA (TLC1) in a way that depends on the finger subdomain. We propose that, in addition to its catalytic role, the finger subdomain of Est2p facilitates the action of Pif1p at telomeres.

Subtelomeric factors antagonize telomere anchoring and Tel1-independent telomere length regulation.

Yeast telomeres are anchored at the nuclear envelope (NE) through redundant pathways that require the telomere-binding factors yKu and Sir4. Significant variation is observed in the efficiency with which different telomeres are anchored, however, suggesting that other forces influence this interaction. Here, we show that subtelomeric elements and the insulator factors that bind them antagonize the association of telomeres with the NE. This is detectable when the redundancy in anchoring pathways is compromised. Remarkably, these same conditions lead to a reduction in steady-state telomere length in the absence of the ATM-kinase homologue Tel1. Both the delocalization of telomeres and reduction in telomere length can be induced by targeting of Tbf1 or Reb1, or the viral transactivator VP16, to a site 23 kb away from the TG repeat. This correlation suggests that telomere anchoring and a Tel1-independent pathway of telomere length regulation are linked, lending a functional significance to the association of yeast telomeres with the NE.

Subtelomeric proteins negatively regulate telomere elongation in budding yeast.

The Tbf1 and Reb1 proteins are present in yeast subtelomeric regions. We establish in this work that they inhibit telomerase-dependent lengthening of telomere. For example, tethering the N-terminal domain of Tbf1 and Reb1 in a subtelomeric region shortens that telomere proportionally to the number of domains bound. We further identified a 90 amino-acid long sequence within the N-terminal domain of Tbf1 that is necessary but not sufficient for its length regulation properties. The role of the subtelomeric factors in telomere length regulation is antagonized by TEL1 and does not correlate with a global telomere derepression. We show that the absence of TEL1 induces an alteration in the structure of telomeric chromatin, as defined biochemically by an increased susceptibility to nucleases and a greater heterogeneity of products. We propose that the absence of TEL1 modifies the organization of the telomeres, which allows Tbf1 and Reb1 to cis-inhibit telomerase. The involvement of subtelomeric factors in telomere length regulation provides a possible mechanism for the chromosome-specific length setting observed at yeast and human telomeres.

The number of vertebrate repeats can be regulated at yeast telomeres by Rap1-independent mechanisms.

The number of telomeric DNA repeats at chromosome ends is maintained around a mean value by a dynamic balance between elongation and shortening. In particular, proteins binding along the duplex part of telomeric DNA set the number of repeats by progressively limiting telomere growth. The paradigm of this counting mechanism is the Rap1 protein in Saccharomyces cerevisiae. We demonstrate here that a Rap1-independent mechanism regulates the number of yeast telomeric repeats (TG(1-3)) and of vertebrate repeats (T(2)AG(3)) when TEL1, a yeast ortholog of the human gene encoding the ATM kinase, is inactivated. In addition, we show that a T(2)AG(3)-only telomere can be formed and maintained in humanized yeast cells carrying a template mutation of the gene encoding the telomerase RNA, which leads to the synthesis of vertebrate instead of yeast repeats. Genetic and biochemical evidences indicate that this telomere is regulated in a Rap1-independent manner, both in TEL1 and in tel1Delta humanized yeast cells. Altogether, these findings shed light on multiple repeat-counting mechanisms, which may share critical features between lower and higher eukaryotes.