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 template for telomere DNA synthesis and may also be important for enzyme structure or catalysis. We have used the presence of conserved sequence elements in the promoter and template regions to amplify by PCR the telomerase RNA genes from six different hypotrichous ciliates: Oxytricha nova, Oxytricha trifallax, Stylonychia mytilis, Stylonychia lemnae, Euplotes aediculatus, and Euplotes eurystomus. RNaseH cleavage of the O. nova RNA in extracts by use of a complementary oligonucleotide leads to loss of telomerase activity, supporting the identification. Primary sequence and biochemical experiments suggest that the templates of Oxytricha and Stylonychia are circularly permuted relative to that of E. aediculatus. On the basis of the pause sites, the former two add G4T4 during a single primer elongation cycle, whereas E. aediculatus adds G3T4G. The only primary sequence element outside the template that is conserved between these phylogenetically distant telomerase RNAs is the sequence 5'-(C)UGUCA-3', which precedes the template regions by exactly two bases. We propose a common secondary structure model that is based on nucleotide covariations, a model which resembles that proposed previously for tetrahymenine telomerase RNAs.