PROGRAMMED TRANSLATIONAL FRAMESHIFTING IN A GENE REQUIRED FOR YEAST TELOMERE REPLICATION

Background: Telomeres are replicated in most eukaryotes by the enzyme telomerase, a specialized reverse transcriptase. A genetic screen in S accharomyces cerevisiae designed to detect telomerase components previ ously led to the identification of four EST ('ever shorter telomeres') genes which are required for telomerase function in vivo. This report describes the cloning and characterization of EST3. Results: We ident ified a potential site of +1 ribosomal frameshifting in the EST3 codin g sequence and demonstrated that translation both upstream and downstr eam of this site is required for EST3 function. Mutation of EST3 such that it could not frameshift resulted in a strain with the same phenot ype as an est3 null mutant, showing that EST3 frameshifting is require d for telomere replication. Immunoblot analysis revealed that two prot eins were synthesized from EST3: a truncated protein resulting from tr anslation of only the first open reading frame, as well as the full-le ngth 181amino-acid Est3 protein resulting from translation through the frameshift site. Only the full-length Est3 protein was required for n ormal EST3 function. Conclusions: A programmed translational frameshif ting mechanism similar to that used by yeast retrotransposons is emplo yed to produce full-length EstS protein. This is the first example in yeast of a cellular gene that uses frameshifting to make its protein p roduct, and a potential link is suggested between retrotransposition a nd the telomerase pathway for telomere maintenance.