Amino Acid Starvation Enhances Programmed Ribosomal Frameshift in Metavirus Ty3 ofSaccharomyces cerevisiae

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doi:10.1155/2016/1840782

Abstract

Ty3 is a retroviral-like element and propagates with a retroviral-like mechanism within the yeast cells. Ty3 mRNA contains two coding regions, which are GAG3 and POL3. The coding region POL3 is translated as a GAG3-POL3 fusion protein by a +1 programmed frameshift. In this study, it was shown that the Ty3 frameshift frequency is significantly increased by amino acid starvation in a Gcn2p complex dependent manner. When the yeast cells were subjected to amino acid starvation, the frameshift frequency of Ty3 increased more than 2-fold in the wild-type yeast cells, mostly independent of Gcn4p. However, Ty3 frameshift frequency remained at basal level in thegcn1,gcn20, orgcn2mutant yeast cells in amino acid starved yeasts. Gcn1p forms a complex with Gcn2p and Gcn20p and is involved in the sensing of uncharged tRNAs on the ribosomal A-site during translation. Increases in uncharged tRNA levels due to amino acid depletion lead to ribosomal pauses. These ribosomal pauses are significant actors in the regulation of Ty3 frameshift frequency. Results of this research revealed that frameshift frequency in Ty3 is regulated by the Gcn2p complex in response to amino acid starvation in yeast.

Highlights

Ty elements of the yeast Saccharomyces cerevisiae belong to the retrotransposon family of eukaryotic mobile genetic elements [1]
The programmed ribosomal frameshifting (PRF) in Ty3 elements is caused by the slow decoding of AGU codon by tRNA-Ser-GCU followed by the out-of-frame binding of tRNA-Val-GUU at +1 direction in the frameshift site of Ty3 [5]
The PRF rate is approximately 4-5% in Ty3, while it is 25% in Ty1 under standard growth conditions in most of the laboratory strains of S. cerevisiae [8, 9]

Summary

Introduction

Ty elements of the yeast Saccharomyces cerevisiae belong to the retrotransposon family of eukaryotic mobile genetic elements [1]. Ty3 is one of the five different retroviral-like elements that are present in the yeast S. cerevisiae genome [2]. Ty3 genome encodes a single mRNA that contains two overlapping coding regions [2]. These coding regions are named TY3A (GAG3) and TY3B (POL3), and they encode retroviral gag and pol analogs, respectively. Posttranslational cleavage of GAG3 generates nucleocapsid proteins that are required for the formation of TY virus like particles (Ty-VLP). Posttranslational proteolytic cleavage of POL3 gives three different proteins that show protease (PR), integrase (IN), and reverse transcriptase/RNAse H (RT/RH) activities [7]. The rate of PRF shows variations among retroviruses and retroviral-like elements [6]. The PRF rate is approximately 4-5% in Ty3, while it is 25% in Ty1 under standard growth conditions in most of the laboratory strains of S. cerevisiae (in synthetic complete medium supplemented with 2% glucose in logarithmically grown cells at 30∘C) [8, 9]

Methods

Results

Discussion

Conclusion