Effects on telomerase activity of the 5'-terminal region of human telomerase RNA.

Human telomerase RNA (hTR) is assumed to contain a 17-nt segment in a single-stranded state at the 5'-terminus. There are four stretches of consecutive G residues, which are apt to form a quadruplex structure, in the segment. It is reported that deletion of some part of this region enhances telomerase activity when the core telomerase enzyme is reconstituted by addition of telomerase reverse transcriptase. To elucidate the reason for such effects, we constructed hTR mutants with deletion of the segment (hTRdelta20) and with the segment containing four G-to-A displacement to interrupt the G-stretch sequences (hTR17A) and their activity was compared with that of the wild-type hTR (hTRW). hTRdelta20 showed much higher activity than the other while hTR17A showed activity similar to that of hTRW. This result suggests that the lower activity for full-length hTR is not due to putative quadruplex formation.