Deficiency of 1-Methylguanosine in tRNA from Salmonella typhimurium Induces Frameshifting by Quadruplet Translocation

Abstract

The trmD gene encodes the tRNA(m1G37)methyltransferase, which methylates guanosine (G) to 1-methylguanosine (m1G) at position 37 of tRNAs that read CUN (leucine), CCN (proline), and CGG (arginine) codons. A mutant, trmD3, has previously been isolated, which at high temperature lacks m1G in tRNA, and this deficiency was correlated with a +1 frameshifting activity. In this study, the mechanism of this trmD3 -induced frameshift involving mutant tRNAPro and tRNALeu species has been investigated. Potential frameshifting sites for proline tRNAs, CCC-N, were efficiently suppressed in the mutant strain. Hybrid β-galactosidases encoded by plasmid constructs containing the sites CCC-U and CCC-A were subjected to amino-terminal sequencing. The protein sequences demonstrated that a quadruplet translocation had occurred and that a proline was inserted at these sites, suggesting that a tRNAPro deficient in m1G is the frameshifting agent. Therefore, a mechanism involving a quadruplet codon-anticodon interaction is favoured for trmD3 -dependent +1 frameshifting. Of the four potential sites for tRNALeu (CCU-N), two, CCU-U and CCU-C, were significantly suppressed in the trmD3 mutant. Thus, species of tRNALeu may also act as +1 frameshift suppressors. No - 1 frameshifting activity was found with the trmD3 mutant.