Altered tRNA processing is linked to a distinct and unusual La protein in Tetrahymena thermophila

La proteins are conserved factors critical for the maturation of RNA polymerase III transcripts. In the ciliate T. thermophila and related alveolates, La proteins have a novel domain arrangement and are linked to a distinct pre-tRNA processing pathway. Nascent pre-tRNAs are transcribed by RNA polymerase III and immediately bound by La proteins on the UUU-3'OH sequence, using a tandem arrangement of the La motif and an adjacent RNA recognition motif-1 (RRM1), resulting in protection from 3'-exonucleases and promotion of pre-tRNA folding. The Tetrahymena thermophila protein Mlp1 has been previously classified as a genuine La protein, despite the predicted absence of the RRM1. We find that Mlp1 functions as a La protein through binding of pre-tRNAs, and affects pre-tRNA processing in Tetrahymena thermophila and when expressed in fission yeast. However, unlike in other examined eukaryotes, depletion of Mlp1 results in 3'-trailer stabilization. The 3'-trailers in Tetrahymena thermophila are uniquely short relative to other examined eukaryotes, and 5'-leaders have evolved to disfavour pre-tRNA leader/trailer pairing. Our data indicate that this variant Mlp1 architecture is linked to an altered, novel mechanism of tRNA processing in Tetrahymena thermophila.

Automated summary

Studies have shown that the Tetrahymena thermophila protein Mlp1 plays a key role in La protein function, affecting pre-tRNA processing and stability of 3'-trailers when depleted. The short length of 3'-trailers in Tetrahymena thermophila compared to other eukaryotes, along with the evolution of 5'-leaders disfavoring pre-tRNA leader/trailer pairing, indicate a unique mechanism of tRNA processing in Tetrahymena thermophila associated with the variant architecture of Mlp1.