FIG. 7.

Models for the biogenesis of yeast telomerase. (A) Shuttling model. Tlc1 would travel to the cytoplasm where it would probably pick up some proteins. Its nuclear reimport would be mediated by Mtr10p. Once in the nucleus the processing of Tlc1 would continue with the assembly of the Sm proteins, which would trigger the cap hypermethylation and 3′ end trimming. Note that the Sm proteins would follow a different transport pathway (2). (B) Processing enzyme model. Tlc1 may or may not travel to the cytoplasm, but its reimport would not depend on Mtr10p. Instead, Mtr10p is transporting to the nucleus an enzyme performing an essential function for proper telomerase biogenesis. A₈₀ denotes the poly(A)⁺ tail of Tlc1 (≈80 nt long) (4).