FIG. 5.

Model of the S. cerevisiae telomerase reaction cycle. (State 1) Telomerase before substrate binding. Template positions ⁴⁷⁹CAC⁴⁷⁷ and ⁴⁷³CAC⁴⁷¹ are not accessible for alignment. (State 2) Upon substrate binding, a conformational change occurs and all template positions are available for base pairing during reverse transcription. Dissociation of a partially extended telomere occurs with moderate frequency. (State 3) The central part of the TLC1 template region is reverse transcribed processively. The results presented in this paper exclude product dissociation at position ⁴⁷⁶C. However, the region of processive reverse transcription may cover template positions ⁴⁷⁸A to ⁴⁷²A to favor incorporation of the telomeric 5′-TGGGTGT-3′ core sequence. (State 4) Template positions close to the 5′ boundary are reverse transcribed with moderate frequency. Only half of the products are extended beyond template position ⁴⁷¹C.