Telomerase repeat addition processivity is increased at critically short telomeres in a Tel1-dependent manner in Saccharomyces cerevisiae

Abstract

Telomerase is the ribonucleoprotein enzyme that elongates telomeres to counteract telomere shortening. The core enzyme consists of a reverse transcriptase protein subunit and an RNA subunit. The RNA subunit contains a short region that is used as a template by the reverse transcriptase to add short, tandem, G-rich repeats to the 3 ends of telomeres. By coexpressing two RNA subunits that differ in the telomeric repeat sequence specified and examining the telomere extensions after one cell cycle, we determined that Saccharomyces cerevisiae telomerase can dissociate and reassociate from a given telomere during one cell cycle. We also confirmed that telomerase is nonprocessive in terms of telomeric repeat addition. However, repeat addition processivity is significantly increased at extremely short telomeres, a process that is dependent on the ATM-ortholog Tel1. We propose that this enhancement of telomerase processivity at short telomeres serves to rapidly elongate critically short telomeres.[Keywords: Telomerase; processivity; Saccharomyces cerevisiae; TLC1; Tel1] Supplemental material is available at http://www.genesdev.org.

Telomerase caught in the act

“…Indeed, while yeast telomerase does not show any processivity in vitro, it is known that during one round of telomere elongation, multiple telomerase molecules can extend a single telomere. 26 Consistent with this view, the T-Recs may favor the action of multiple telomerase molecules on a single telomere. Such a mechanism may ensure an extensive elongation of a given telomere by increasing the number of engaged telomerase complexes and thus explain how a single telomere can gain up to hundred nucleotides in one cell cycle.…”

PP2A and Aurora differentially modify Cdc13 to promote telomerase release from telomeres at G2/M phase

“…Several laboratories also discovered genetic and physical evidence of the importance of Tel1 in telomerase recruitment [23][24][25] . In this study, our phospho-specific antibody detected in vivo S249 and S255 phosphorylation in a Tel1/Mec1-dependent and cell cycle-dependent manner and the signal was abolished in the cdc13 S249S255A cells.…”

“…Telomere replication is facilitated by Cdk1/Tel1/Mec1-mediated Cdc13 phosphorylation at late S phase [22][23][24][25][26][27] . G2 phase is well known as the pre-mitotic phase to prepare for M phase entry.…”

“…1a) [22][23][24][25][26][27] . To examine the possibility that recruitment of telomerase by Cdc13 may be repressed by phosphatases, we determined the phosphorylation level of Cdc13 throughout the cell cycle for wild type (WT) and various phosphatase deletion strains.…”

“…It recruits telomerase to its site of action through an electrostatic interaction between Cdc13 and Est1 (refs 20,21). Some recent studies have suggested that Cdk1, Tel1 and Mec1 may phosphorylate the telomerase recruitment domain of Cdc13 to promote this Cdc13-Est1 interaction at the late S phase [22][23][24][25][26][27] .…”