Eukaryotic wobble uridine modifications promote a functionally redundant decoding system

Department of Molecular Biology, Umeå University, 901 87 Umeå, Sweden

^* To whom correspondence should be addressed. Email: Anders.Bystrom{at}molbiol.umu.se.

	Abstract

The translational decoding properties of tRNAs are modulated by naturally occurring modifications of their nucleosides. Uridines located at the wobble position (nucleoside 34) in eukaryotic cytoplasmic tRNAs often harbor a 5-methoxycarbonylmethyl (mcm⁵) or a 5-carbamoylmethyl (ncm⁵) side-chain and sometimes an additional 2-thio (s²) or 2'-O-methyl group. Although a variety of models explaining the role of these modifications have been put forth, their in vivo functions have not been defined. In this study, we utilized recently characterized modification-deficient yeast cells to test the wobble rules in vivo. We show that mcm⁵ and ncm⁵ side-chains promote decoding of G-ending codons and that concurrent mcm⁵ and s² groups improve reading of both A- and G-ending codons. Moreover, the observation that the mcm⁵U₃₄- and some ncm⁵U₃₄-containing tRNAs efficiently read G-ending codons challenges the notion that eukaryotes do not use U-G wobbling.