The functionality of telomerase depends on CPF–CF induced 3′end processing of its RNA component <i>TLC1</i> and a novel Nrd1–Nab3 surveillance mechanism

Telomere elongation is driven by telomerase, which consists of several proteins and the ncRNA component TLC1 in yeast. While many ncRNAs are terminated via the Nrd1-Nab3-Sen1 (NNS) pathway, we found that TLC1 requires cleavage and polyadenylation factor (CPF)-cleavage factor (CF) mediated 3'end processing and the resulting poly(A) tail to mature into a functional ribozyme. The poly(A) tail is predicted to fold back onto (U)-repeats potentially forming a terminal stem-loop structure that supports Sm-ring binding and thereby re-import into the nucleus after cytoplasmic shuttling. However, longer pre-TLC1 transcripts are predicted to fold differently, resulting in Sm-ring and import receptor binding defects, leaving them unable to overcome this cytoplasmic quality control checkpoint. To prevent cytoplasmic leakage of overlong transcripts, we propose an additional nuclear monitoring system, requiring Nrd1-Nab3 binding sites located between the first PAS motifs. CPF-CF formation might compete with Nrd1-Nab3 releasing them from shorter but not from longer transcripts facilitating their decay. This potential competitive RNA-binding of CPF-CF and Nrd1-Nab3 balances stability and decay.