Seven, eight and nine-membered anticodon loop mutants of tRNA 2Arg which cause +1 frameshifting : Tolerance of DHU arm and other secondary mutations

Abstract

The mutant tRNAf 2 Arg encoded by the genetically-selected frameshift suppressor, sufT621, inserts arginine and causes a +1 reading-frame shift at the proline codon, CCG(U). There is an extra base, G36.1, in argV β , one of the four identical genes for tRNA 2 Arg in the position between bases 36 and 37, corresponding to the 3′ side of the anticodon. The new four-base anticodon, predicted from DNA sequencing to be 3′ GGCA 5′, is complementary to the four-base codon CCGU. Quadruplet translocation promoted by mutant argV does not require perfect complementarity between the codon and the anticodon since synthetic genes encoding derivatives of tRNA 2 Arg and tRNA 1 Pro, with four-base anticodons complementary to three out of the four bases of CCGU, were also shown to be capable of frameshifting. Two other mutants of argV, inferred to have normal-size, seven-base anticodon loops, were also found to be capable of four-base-decoding demonstrating that quadruplet translocation promoted by mutant argV does not require an enlarged anticodon loop. Other alleles of argV, predicted to have nine bases in the anticodon loop, were also found to cause frameshifting. The DNA sequence of two of these showed in addition, either a deletion of G24, or a ten-base duplication in the region corresponding to the TFC arm. A general finding is that mutations in the DHU arm of tRNA 2 Arg are compatible with, and in one case necessary for, frameshifting.