FIGURE 4.

S. pombe telomerase RNP biogenesis. S. pombe TER1 is transcribed by RNA polymerase II as a precursor that includes a downstream intron and exon. TER1 secondary structure has yet to be experimentally validated; it is illustrated here as similar to other yeast TERs. Sm proteins assemble on the precursor near the mature TER1 3′ end. Spliceosomal cleavage at the 5′ splice site releases mature TER1, which escapes ligation to the downstream exon. Sm proteins then recruit the cap hypermethylase Tgs1, which modifies the 5′ cap to TMG, indicated by a diamond. Sm proteins are then replaced by Lsm proteins that protect the 3′ end from degradation, followed by assembly of Est1 and the TERT subunit Trt1.