Rare transfer ribonucleic acid essential for phage growth. Nucleotide sequence comparison of normal and mutant T4 isoleucine-accepting transfer ribonucleic acid

Abstract

One of the eight tRNA species coded by bacteriophage T4 is unique in that (1) it is found in a yield lower by three- to fourfold than that of any other tRNA and (2) while dispensable for growth in standard laboratory hosts, it is essential for phage propagation in a natural isolate of Escherichia coli (strain CT439). We report here the nucleotide sequence of this tRNA and of several mutationally altered forms. The molecule is 77 nucleotides in length and has the anticodon N-A-U. Depending on the pairing properties of the "wobble" nucleotide N, this sequence could correspond to one or more of the isoleucine-specific codons (formula: see text) or to the methionine-specific codon A-U-G. Since a T4-specific acceptor activity for isoleucine which is stimulated in ribosome binding by A-U-A but not A-U-U has been reported previously, we infer that we have sequenced a tRNA Ile species which preferentially recognizes A-U-A. Mutant HA1 is unable to grow in CT439; it produces no tRNA Ile. The primary mutational alteration is a transition four residues from the 5'terminus which converts a C.G to a U.G base pair. The consequences of this lesion can be partially reversed by second-site mutations nearby in the acceptor stem. Unexpectedly, the tRNA Ile synthesized in these revertants still retains two unusual structural features found in the wild-type molecule: the opposition of two Up residues in the amino acid acceptor stem and the opposition of an Ap and a Gp residue in the anticodon stem. Implications of these structual anomalies for a possibly unique physiological role of this minor tRNA species are discussed.