318 Telomerase and the maintenance of chromosome ends Tracy M Bryan* and Thomas R Cech? The catalytic subunit of telomerase has recently been identified in diverse eukaryotes and shown to be a reverse transcriptase. &topic expression of this protein in normal human cells leads to lengthened telomeres and an extended in vitro life span. Other proteins that modulate telomerase activity in viva are also being identified, including a functionally conserved family of proteins with Myb-like DNA-binding domains and proteins that are involved in DNA double-strand break repair. Addresses Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, Campus box 215, University of Colorado, Boulder, CO 80309-0215, USA *e-mail: tracy.bryan@colorado.edu ie-mail: thomas.cech@colorado.edu Current Opinion in Cell Biology 1999, 11:318-324 http://biomednet.com/elecref/O955067401 100318 G Elsevier Science Ltd ISSN 0955-0674 Abbreviations EST2 Ever shorter telomeres 2 PARP poly(adenosme diphosphate-ribose) polymerase PD population doublings RT reverse transcriptase TERT telomerase RT TRFl telomeric repeat binding factor 1 Introduction ‘Ii-lomeres, which form a protective cap on chromosome ends, are usually composed of short G-rich DNA repeats complexed with proteins. ‘I’he replication of celomeres involves the ribonucleoprotein enzyme telomerase- a revcrsr transcriptase (RI’) that usesan intrinsic RNA as a template for the extension of the G-rich strand of the tclomere [l]. ‘I’his revie\v discusses the current state of our knowledge about the protein components of telomerase Figure 1 and telomeres. The evidence that the maintenance of telomeres is required for continued cell proliferation is also considered briefly. There has been an explosion of telom- ere research over the past two years, only a portion of which can be covered in this short review. Components of the telomerase enzyme Telomeraseactivity depends on an essential RNA subunit. The gene for this RNA has been cloned from more than 20 species of ciliated protozoa, the yeasts S’~~%laro~z~v~rs (7re- cisiar and K&yoeronzyre.s Lz&, mice and humans (reviewed in [Z]). These RNAs all contain a sequence that is comple- mentary to about 1.5 t&mere repeats and acts as the template for telomere elongation. Regions of telomerase RNAs outside the template are beginning to be explored and have been found to contribute to telomerase active site functions aswell asassembly of the ribonucleoprotein complex [3,4]. ‘I’he past two years have witnessed the identification of the long sought-after catalytic protein subunit of telomerase. Biochemical fractionation of cell extracts from the ciliate kup/ote.r cle&&rt~~.s revealed two proteins of 12.3 kDa and 4.3 kDa that co-purify with telomerase activity [.5]. When the gene for ~123 wascloned, it was found to be the homo- loguc of the X. rerwisi~~e gene I?‘ST? (E:zlef- shortu/- telo~~w-e.s) [h”], deletion of which had previously been shown to pro- duce shortened telomeres and senescence [7]. ‘l’hc derived amino acid sequencesof ~1.23and EstZp contain motifs common to all R’lk, consistent with the role of telomerase in RNA-templated DNA polymerisation. Homologues of the telomerase R’I’ (‘IXR’I’) have since been identified in humans, mice, fission yeast and the cil- i a te s Etrahymnla thermophilll and Oxytrkhu tr~fhlhx (Figure 1) [8’,9’,10-141. Mutation of the three aspartic CP T12 A B’CD E a Tetrahymena thermophila Oxytricha triiallax Euplotes aediculatus Saccharomyces cerevrsiae Schizosaccharomyces pombe Homo sapiens 1 I Mus musculus Current Opmon in Cell Bcalogy Primary structure of the seven known telomerase reverse transcriptases. [W,9*]) and Mus musculus (mTERT; [14]). Shaded boxes indicate the Proteins represented are from Tefrahymena thermophila (Tt-TERT; locations of reverse transcriptase motifs 1,2 and A-E, the telomerase- [12,13]), Oxyfricha frifakx (Ot-TERT; [12]), Euplofes aediculatus specific motif T [8’], and a putative motif of unknown function found in (Ea-~123; [6”1), Saccharomyces cemvisiae (Sc-Est2p; [7]), clliated protozoa, CP [i 21. Functions of regions of the proteins outside Schizosaccharomyces pombe (Sp-Trtl p; IS.]), Homo sapiens (hTERT; the RT motifs have yet to be determined. aa, amino acids.