THE EFFECTS OF NUCLEOSIDE ANALOGS ON TELOMERASE AND TELOMERES IN TETRAHYMENA

The ribonucleoprotein enzyme telomerase is a specialized type of cellu lar reverse transcriptase which synthesizes one strand of telomeric DN A, using as the template a sequence in the RNA moiety of telomerase. W e analyzed the effects of various nucleoside analogs, known to be chai n-terminating inhibitors of retroviral reverse transcriptases, on Tetr ahymena thermophila telomerase activity in vitro. We also analyzed the effects of such analogs on telomere length and maintenance in vivo, a nd on vegetative growth and mating of Tetrahymena cells. Arabinofurany l-guanosine triphosphate (Ara-GTP) and ddGTP both efficiently inhibite d telomerase activity in vitro, while azidothymidine triphosphate (AZT -TP), dideoxyinosine triphosphate (ddITP) or ddTTP were less efficient inhibitors. All of these nucleoside triphosphate analogs, however, pr oduced analog-specific alterations of the normal banding patterns seen upon gel electrophoresis of the synthesis products of telomerase, sug gesting that their chain terminating and/or competitive actions differ at different positions along the RNA template. The analogs AZT, 3'-de oxy-2',3'-didehydrothymidine (d4T) and Ara-G in nucleoside form caused consistent and rapid telomere shortening in vegetatively growing Tetr ahymena. In contrast, ddG or ddl had no effect on telomere length or c ell growth rates. AZT caused growth rates and viability to decrease in a fraction of cells, while Ara-G had no such effects even after sever al weeks in culture. Neither AZT, Ara-G, acycloguanosine (Acyclo-G), d dG nor ddl had any detectable effect on cell mating, as assayed by qua ntitation of the efficiency of formation of progeny from mated cells. However, AZT decreased the efficiency of programmed de novo telomere a ddition during macronuclear development in mating cells.