Telomerase holoenzymes

Abstract

Telomerase is unusual genome replication machinery. The eukaryotic reverse transcriptase ribonucleoprotein (RNP) supplements telomeric simple‐sequence repeat tracts at chromosome ends by new repeat synthesis. Using each chromosome 3’ end as primer and an internal region of the integral telomerase RNA subunit as template, telomerase reiteratively reverse transcribes its short template into tandem single‐stranded telomeric repeats. Lagging‐strand DNA replication machinery and processing enzymes then create the functional telomere structure of duplex repeats with a 3’ overhang that can be single‐stranded, form alternative DNA structures such as G‐quadruplex, or invade the duplex region to form a displacment loop (D‐loop) of telomeric repeats termed the t‐loop.Two and a half decades of research in the Collins lab led the way to a resolution for several puzzles that were facing the telomere and telomerase field since soon after telomerase discovery by Carol Greider and Elizabeth Blackburn. How do telomerase reverse transcriptase protein (TERT) and telomerase RNA co‐fold in a hierarchical, complex process of cellular telomerase RNP biogenesis, and how does this drastically limit the amount of telomerase made by even the most strongly telomerase‐positive human cancer and stem cells? How can a precisely defined region within telomerase RNA be used as template? How does the template‐product duplex unpair and reform in a different template register to enable processive repeat synthesis? How is telomerase brought to telomeres and how does each telomere control telomerase activity according to its individual length to generate telomere length homeostasis?Human telomerase RNA (hTR) relies on proteins shared with H/ACA‐family small nucleolar RNAs for its processing, stability, and assembly with TERT. Inherited and spontaneous mutations in these H/ACA proteins cause disease syndromes by reducing telomerase function, as do subsequently identified mutations in hTR and TERT. Understanding of how these mutations compromise telomerase function has emerged from new high‐resolution structures of telomerase holoenzymes from the ciliate Tetrahymena thermophila and human cells, the two model systems of Collins lab research, and from understanding control of telomerase function at telomeres.