THERMALLY-INDUCED DNA-CENTER-DOT-RNA HYBRID TO G-QUADRUPLEX TRANSITIONS - POSSIBLE IMPLICATIONS FOR TELOMERE SYNTHESIS BY TELOMERASE

Telomerase is a specialized reverse transcriptase that contains its ow n RNA template for synthesis of telomeric DNA [Greider, C, W., & Black burn, E, H, (1989) Nature 337, 331-337; Shippen-Lentz, D., & Blackburn , E. H, (1990) Science 247, 546-552], The activity of this ribonucleop rotein enzyme has been associated with cancer cells [Kim et al. (1994) Science 266, 2011-2015] and is thus a potential target for anticancer chemotherapy. Telomeric DNA . RNA hybrids are important intermediates in telomerase function and form after extension of the growing telome re on the telomerase RNA template, Translocation is a critical step in telomerase function and consists of unwinding of the telomeric DNA te lomerase RNA hybrid followed by repositioning of the 3'-end of the ext ended telomere. A central question in telomerase function is how trans location of the extended telomere occurs in the absence of ATP or GTP. It has been hypothesized that unwinding of the telomeric hybrid may b e facilitated by the formation of stable hairpins or G-quadruplexes by the telomere product (i.e., a hybrid to G-quadruplex transition) and that this may provide at least part of the driving force for transloca tion [Shippen-Lentz & Blackburn, 1990; Zahler et al, (1991) Nature 350 , 718-720]. However, so far there has been no effort aimed at examinin g the possibility that a hybrid/G-quadruplex equilibrium can occur and to what extent this equilibrium depends on buffer and concentration c onditions. Examination of these transitions may provide insight into t elomerase function and may also provide clues for the development of a nti-telomerase agents. Using a model system consisting of the DNA . RN A hybrid d(GGTTAGGGTTAG). r(cuaacccuaacc), we present evidence that a thermally induced transition of telomeric DNA . RNA hybrid to G-quadru plex can occur under certain conditions, These results provide support for the hypothesis that G-quadruplex formation by the telomere produc t may in fact regulate telomerase function at the translocation step ( Zahler et al., 1991) and suggest an Achilles' heel for indirectly targ eting telomerase. Thus, on the basis of the insight gained from the pr esent studies and the results of Zahler et al. (1991), we propose that ligands that selectively bind or cleave G-quadruplex structures may m odulate telomerase processivity.