A unique pause pattern during telomere addition by the error-prone telomerase from the ciliate Paramecium tetraurelia

Molecular and Evolutionary Analysis of RNA-Protein Interactions in Telomerase Regulation

Telomerase is an enzyme involved in the maintenance of telomeres. Telomere shortening due to the end-replication problem is a threat to the genome integrity of all eukaryotes. Telomerase inside cells depends on a myriad of protein-protein and RNA-protein interactions to properly assemble and regulate the function of the telomerase holoenzyme. These interactions are well studied in model eukaryotes, like humans, yeast, and the ciliated protozoan known as Tetrahymena thermophila. Emerging evidence also suggests that deep-branching eukaryotes, such as the parasitic protist Trypanosoma brucei require conserved and novel RNA-binding proteins for the assembly and function of their telomerase. In this review, we will discuss telomerase regulatory pathways in the context of telomerase-interacting proteins, with special attention paid to RNA-binding proteins. We will discuss these interactors on an evolutionary scale, from parasitic protists to humans, to provide a broader perspective on the extensive role that protein-protein and RNA-protein interactions play in regulating telomerase activity in eukaryotes.

Programmed chromosome fragmentation in ciliated protozoa: multiple means to chromosome ends

SUMMARYCiliated protozoa undergo large-scale developmental rearrangement of their somatic genomes when forming a new transcriptionally active macronucleus during conjugation. This process includes the fragmentation of chromosomes derived from the germline, coupled with the efficient healing of the broken ends by de novo telomere addition. Here, we review what is known of developmental chromosome fragmentation in ciliates that have been well-studied at the molecular level (Tetrahymena, Paramecium, Euplotes, Stylonychia, and Oxytricha). These organisms differ substantially in the fidelity and precision of their fragmentation systems, as well as in the presence or absence of well-defined sequence elements that direct excision, suggesting that chromosome fragmentation systems have evolved multiple times and/or have been significantly altered during ciliate evolution. We propose a two-stage model for the evolution of the current ciliate systems, with both stages involving repetitive or transposable elements in the genome. The ancestral form of chromosome fragmentation is proposed to have been derived from the ciliate small RNA/chromatin modification process that removes transposons and other repetitive elements from the macronuclear genome during development. The evolution of this ancestral system is suggested to have potentiated its replacement in some ciliate lineages by subsequent fragmentation systems derived from mobile genetic elements.

Biochemical properties of Trypanosoma cruzi telomerase

Trypanosomatid parasite infections have a devastating impact on human health. Little is known about the requirements for parasite growth during any stage of their complex, multi-host life cycle. In most eukaryotic organisms, sustained cell proliferation requires telomerase-dependent telomere length maintenance. Here we investigate the regulation and biochemical features of telomerase from Trypanosoma cruzi, the causative agent of Chagas disease. We found that T.cruzi telomerase is active in extracts from multiple developmental stages of the parasite life cycle. Detailed characterization of the enzymatic properties of telomerase using epimatigote-stage extract revealed a unique combination of substrate specificities, consistent with the evolutionary divergence of trypanosomes from previously established model systems for telomerase biochemical characterization. Results from partial purification of T.cruzi telomerase suggest that the catalytically active enzyme is a large ribonucleoprotein complex and that the internal RNA template has an atypical, cytosine-rich permutation. These results expand our understanding of telomerase enzymology and should encourage the development of parasite-specific telomerase inhibitors as a method for disease therapy.