TELOMERASE RNAS OF DIFFERENT CILIATES HAVE A COMMON SECONDARY STRUCTURE AND A PERMUTED TEMPLATE

Telomerase is composed of protein and RNA. The RNA serves as a templat e for telomere DNA synthesis and may also be important for enzyme stru cture or catalysis. We have used the presence of conserved sequence el ements in the promoter and template regions to amplify by PCR the telo merase RNA genes from six different hypotrichous ciliates: Oxytricha n ova, Oxytricha trifallax, Stylonychia mytilis, Stylonychia lemnae, Eup lotes aediculatus, and Euplotes eurystomus. RNaseH cleavage of the O. nova RNA in extracts by use of a complementary oligonucleotide leads t o loss of telomerase activity, supporting the identification. Primary sequence and biochemical experiments suggest that the templates of Oxy tricha and Stylonychia are circularly permuted relative to that of B. aediculatus. On the basis of the pause sites, the former two add G(3)T (4) during a single primer elongation cycle, whereas E. aediculatus ad ds G(3)T(4)G. The only primary sequence element outside the template t hat is conserved between these phylogenetically distant telomerase RNA s is the sequence 5'-(C)UGUCA-3', which precedes the template regions by exactly two bases. We propose a common secondary structure model th at is based on nucleotide covariations, a model which resembles that p roposed previously for tetrahymenine telomerase RNAs.