SUMMARY PARAGRAPH for TLC1
Telomerase is a ribonucleoprotein complex that is essential for maintenance of telomeres, special sequences which terminate the ends of linear chromosomes. Telomerase is a reverse transcriptase that elongates the single-stranded G-rich 3' protruding ends of chromosomal DNA using an RNA molecule that is part of the telomerase complex. The extended strand provides a template for synthesis of the lagging strand by DNA polymerase, thus preventing the otherwise inevitable loss of terminal DNA at each round of replication.
In yeast, five gene products are required for telomerase activity in vivo: Est2p (the catalytic reverse transcriptase subunit), TLC1 (the template RNA), Est1p, Est3p and Cdc13p. Mutations in any of these five genes lead to progressive telomere shortening, the so-called ever shorter telomeres (EST) phenotype, followed by cell death. CDC13 is the only essential gene among the EST genes. Est2p and TLC1 form the catalytic core of telomerase, while Est1p, Est3p and Cdc13p which are dispensable for in vitro telomerase catalytic activity, play regulatory roles (3, 4, 5, 6, 7 and references therein). Cdc13p, a single stranded DNA binding protein required for telomere maintenance and elongation, binds to Est1p and this interaction is necessary for recruiting telomerase to the chromosomal ends. Est1p, Est2p and Est3p all bind to the TLC1 RNA template and Est1p also binds to 3' ends of single stranded DNA. Est1p forms a stable complex with TLC1 in the absence of Est2p or Est3p while association of Est3p with the enzyme requires an intact catalytic core. Est1p and Est3p are stable components of the telomerase holoenzyme (6).
TLC1 is the template for the synthesis of the single stranded chromosome end synthesis; it also provides a scaffold for the assembly of the telomerase ribonucleoprotein complex and it modulates enzyme activity (8). TLC1 was identified from a screen for telomeric silencing defects (1). In S. cerevisiae, the TLC1 RNA is a polyadenylated, 1.3 kb transcript produced by RNA polymerase II. An increase in the amount of polyA+ TLC1 transcript is observed between the G1 and S phases of the cell cycle, suggesting that TLC1 RNA levels are regulated in a cell-cycle dependent manner (4).
Analysis of the secondary structure of the TLC1 RNA has provided insights into the RNA-protein interactions that are necessary for the assembly and activity of the telomerase complex. A base-paired element immediately adjacent to the template, provides a template boundary to terminate each cycle of reverse transcription (9) and three stem-loop structures provide protein binding sites for the Est2p, Est1p telomerase subunits and for the Ku heterodimer (Yku70p-Yku80p) (10, 11, 12). Binding of Est1p with TLC1 is proposed to provide a bridge between the catalytic Est2p and the telomere-bound Cdc13p, thereby mediating an essential step in telomere replication (10, 11, 12). Interaction of TLC1 RNA with the Ku dimer promotes the addition of telomeres to broken chromosome ends, thereby repairing damaged DNA by capping the broken end with telomeric DNA (12, 13). The TLC1 RNA also contains an Sm binding site near its 3' end and is bound by the heteroheptameric Sm ring complex, which also binds to many of the splicesomal snRNAs and is encoded by SMB1, SMD1, SMD2, SMD3, SME1, SMX3 and SMX2 (14). It has been proposed that the Sm proteins play a role in the intracellular transport, assembly and maturation of the telomerase RNP complex. Approximately 40% of the length of the TLC1 RNA, including the template region and the Est2p and Est1p binding sites, is required for telomerase function (15).
In humans, telomere length is linked to aging and cancer: in human germline cells telomeres are long, whereas in cells of somatic tissues, telomerase activity is absent and the telomeres are short. Upon sufficient shortening, the somatic cells stop dividing and become senescent. Inappropriate telomerase activity is detected in most malignant tumors, and the genes required for telomerase activity are potential targets for cancer therapy (16, 4).
Human orthologs for four of the telomerase subunits are known. Est2p, the telomerase reverse transcriptase catalytic enzyme, is similar to TERT (OMIM), TLC1, the template RNA is similar to TERC/hTR (OMIM), while Cdc13p shares sequence similarity with human POT1 (OMIM) (17, 5). There are three Est1p like proteins in humans, although only hEST1A and hEST1B have been shown to be associated with the telomerase (18). A human ortholog for EST3 hasn't been identified. Mutations in TERT (OMIM) and TERC/hTR (OMIM) cause short telomeres and congenital aplastic anemia (OMIM, 17).
Last updated: 2007-06-07