The Sua5 Protein Is Essential for Normal Translational Regulation in Yeast

Abstract

The anticodon stem-loop of tRNAs requires extensive posttranscriptional modifications in order to maintain structure and stabilize the codon-anticodon interaction. These modifications also play a role in accommodating wobble, allowing a limited pool of tRNAs to recognize degenerate codons. Of particular interest is the formation of a threonylcarbamoyl group on adenosine 37 (t 6 A 37 ) of tRNAs that recognize ANN codons. Located adjacent and 3 to the anticodon, t 6 A 37 is a conserved modification that is critical for reading frame maintenance. Recently, the highly conserved YrdC/Sua5 family of proteins was shown to be required for the formation of t 6 A 37 . Sua5 was originally identified in a screen by virtue of its ability to affect expression from an aberrant upstream AUG codon in the cyc1 transcript. Together, these findings implicate Sua5 in protein translation at the level of codon recognition. Here, we show that Sua5 is critical for normal translation. The loss of SUA5 causes increased leaky scanning through AUG codons, ؉1 frameshifting, and nonsense suppression. In addition, the loss of SUA5 amplifies the 20S RNA virus found in Saccharomyces cerevisiae, possibly through an internal ribosome entry site-mediated mechanism. This study reveals a critical role for Sua5 and the t 6 A 37 modification in translational fidelity.tRNAs require extensive modification in the anticodon and stem-loop domain (ASL) to function in codon recognition and maintenance of reading frame during translation. The anticodon, consisting of nucleosides 34 to 36, needs to base pair correctly with the codon for accurate translation. Modifications added to nucleosides in the tRNA anticodon region help to stabilize this interaction. Positions 34 and 37 in the ASL are two of the most modified nucleosides (8). Posttranscriptional modifications at wobble position 34, aided by modified nucleoside 37, which is most often a purine, permit noncanonical base pairing to occur at the third base pair. Importantly, this allows wobble to occur and thereby expands the decoding ability of tRNAs (3). Modified nucleosides also serve to minimize conformational movements of the ASL and as such, are important for providing structure to this region (2). This increases the affinity of each tRNA for the cognate codon in the decoding site, providing more free energy for the 30S ribosome to undergo a conformational rearrangement needed to close on the correct codon-anticodon complex for accurate decoding (45,49,59).Purine 37 is a highly conserved nucleoside located 3Ј adjacent to the anticodon that can undergo extensive modifications which are critical for stabilizing codon-anticodon interactions. Of the myriad modifications that exist at position 37, the presence of an N 6 -threonylcarbamoyl group on adenosine 37 (t 6 A 37 ) is one of the most ubiquitous and conserved (22). t 6 A 37 is found in most tRNAs decoding ANN (N representing one of the four canonical nucleotides, A, G, C, or U). Its main role is to create a planar hydrophobic structure that stacks abov...

Conserved residues in yeast initiator tRNA calibrate initiation accuracy by regulating preinitiation complex stability at the start codon

“…Consistent with this, mutations that reduce t 6 A37 formation in yeast impair recognition of AUG codons (Lin et al 2009;Daugeron et al 2011;Srinivasan et al 2011) and can increase the ratio of GUG to AUG initiation . To eliminate the possibility that G31:C39 substitutions confer Sui À phenotypes by impairing t 6 A37 formation, we purified wild-type and G31A:C39U mutant tRNA i , digested them with nuclease P1, and resolved the nucleoside products by high-performance liquid chromatography (HPLC).…”

NMR-based Structural Analysis of Threonylcarbamoyl-AMP Synthase and Its Substrate Interactions

“…By strengthening the codon/anticodon interaction and ribosome entry, t 6 A 37 is believed to participate in the maintenance of the translational reading frame. This is supported by the increase in translational frameshifts in cells lacking the ability to form t 6 A 37 (13)(14)(15). Recent studies of the t 6 A 37 biosynthesis pathway have identified the following four proteins in Escherichia coli that are necessary and sufficient for in vitro formation of the modification: TsaC, TsaD, TsaE, and TsaB, also known as YrdC, YgjD, YjeE, and YeaZ, respectively (16).…”

YrdC exhibits properties expected of a subunit for a tRNA threonylcarbamoyl transferase

“…In this study sua5 deletion mutants led to the loss of t 6 A 37 . Additionally, reading frame maintenance of translation was adversely affected by loss of Sua5 and t 6 A 37 (Lin et al 2010). Several co-crystal structures of RNAmodifying enzymes and their substrate RNAs have been solved, providing information about their mechanisms of action (for review, see Byrne et al 2009).…”

“…Nucleosides 34 and 37 are also two of the most diversely and chemically rich modified nucleosides found in all RNAs (Agris 1996;Jühling et al 2009). These modifications define anticodon architecture, restrict dynamics of the ASL, enhance codon affinity and ribosome binding, as well as reduce translational frameshifting (Urbonavicius et al 2001;Agris 2004;Lin et al 2010).…”