Saccharomyces cerevisiae telomerase is an Sm small nuclear ribonucleoprotein particle

Activation of the chromosome end-replicating enzyme telomerase can greatly extend the lifespan of normal human cells' and is associated with most huma n cancers(2). In all eukaryotes examined, telomerase has an RNA subunit(3), a conserved reverse transcriptase subunit(4) and additional proteins(5,6) but little is known about the assembly of these components. Here we show th at the Saccharomyces cerevisiae telomerase RNA(7) has a 5'-2,2,7-trimethyl- guanosine (TMG) cap and a binding site for the Sm proteins, both hallmarks of small nuclear ribonucleoprotein particles (snRNPs) that are involved in nuclear messenger RNA splicing(8,9). Immunoprecipitation of telomerase from yeast extracts shows that Sm proteins are assembled on the RNA and that mo st or all of the telomerase activity is associated with the Sm-containing c omplex These data support a model in which telomerase RNA is transcribed by RNA polymerase II (ref. 10) and 7-methylguanosine-capped, binds the seven Sm proteins, becomes TMG-capped and picks up the other protein subunits, We conclude that the functions of snRNPs assembled by this pathway are not re stricted to RNA processing, but also include chromosome telomere replicatio n.