The effects of Tma20 and Tma22 on gene expression and telomere function

Abstract

Translation Machinery Association 20 (Tma20) and Tma22 function as interacting partners and are highly conserved. TMA20 and TMA22 are among a diverse range of yeast genes, which have been shown to decrease fitness of cells with cdc13-1 induced telomere defects. Deletions of nonsense mediated decay genes (NMD) also increase fitness of cdc13-1 and it is thought that this is partially due to the higher levels of Stn1 in NMD null strains. Genetic interaction studies show that TMA20 and TMA22 function in the same pathway as NMD genes, but in parallel pathways to DNA damage genes, to affect fitness of cdc13-1. This led us to hypothesise that TMA20 and TMA22 also affect fitness of cdc13-1 by increasing levels of Stn1. Consistent with this, we found that tma20Δ strains do indeed have increased levels of Stn1. In Drosophila Tma20 and Tma20 regulate the expression of genes that contain upstream ORFs (uORFs) by promoting translation re-initiation. However we found no evidence that Tma20 and Tma22 promote translation re-initiation in yeast, as they do in Drosophila. Interestingly though, we observed that STN1 has a uORF and also an ORF that overlaps with the main coding sequence of STN1 which we refer to as an oORF. We demonstrate that the increase in expression of STN1 that occurs upon deletion of TMA20 is dependant on the ORF that overlaps with the main coding sequence of STN1, rather than the uORF. We show that the oORF of STN1 serves as an important regulatory element, which dramatically reduces levels of Stn1. Fitness of cdc13-1 is substantially increased in strains that contain a point mutation in the initiation codon of the oORF. The human homolog of STN1 also has an oORF, suggesting this mechanism of regulating levels of crucial telomere proteins may be conserved.