Biogenesis of telomerase RNA subunit in schizosaccharomyces pombe

Abstract

Telomerase is a multi-subunit ribonucleoprotein complex responsible to replenish the missing telomeric DNA sequence during each round of DNA replication. It has two constitutive members: the protein catalytic subunit (TERT) and the telomerase RNA (TER). The TERT subunit is a reverse transcriptase dependent on TER which has the template region for DNA synthesis and the scaffold sequence for its assembly. Dysregulation of telomerase causes human disorders like dyskeratosis congenital, aplastic anemia among others; and reactivation is observed in ~85% of cancers. Since the clinical relevance of telomerase, it is important to develop strategies to manipulate its function. Here, I use the eukaryote Schizosaccharomyces pombe to understand the biogenesis of telomerase. Previous works have identified the S. pombe telomerase RNA (TER1) precursor is bound by the Sm complex. The Sm complex promotes spliceosomal cleavage (splicing reaction albeit skipping the second transesterification step) and trimethyguanosine (TMG) cap formation. TER1 mature form then transitions from Sm to Lsm2-8 (Sm-like) complex, where the latter stabilize the RNA and promotes binding to the catalytic subunit Trt1. I now demonstrate that an efficient sequential binding from Sm to Lsm requires Pof8, a homolog of ciliate telomeres subunit p65/43. Deletion of pof8 results in decreased Lsm2-8 complex loading onto TER1 mature form which causes an important assembly defect and critically short telomeres. The binding of Pof8 to TER1 is enhanced by at least two other members Bmc1, a methyltransferase, and a Thc1, a previously uncharacterized protein. Deletion of Bmc1 and Thc1 resulted in reduce TER1 levels and shorter telomeres. Overall, the findings in this work provide new insights into the stepwise telomerase biogenesis and suggest a possible regulatory mechanism to coordinate the biogenesis of the 5’ and 3’ of TER1 molecules.