Identification of transfer RNA suppressors in Escherichia coli. III. Ochre suppressors of lysine tRNA.

Transducing phages of lambda carrying suppressors, lysT (Su+ beta), supG and and supL, were isolated in vivo. Upon infection with each of these phages, the production of tRNALys and tRNAVal1 was markedly enhanced. Fingerprint analysis of these tRNAs revealed that they consisted of normal tRNALys, mutant tRNALys and tRNAVal1 in equimolar ratios. The mutant tRNALys carried a single-base alteration at the anticodon, from 5'-UUU-3' to 5'-UUA-3', which makes it an ochre suppressor. DNA sequence analysis of the entire transducing fragment (730 base-pairs) of lambda pSu+ beta revealed that three tRNA genes are tightly clustered within a transcription unit in the following order; i.e. promoter-(48 base-pairs)-wild-type tRNALys-(132 base-pairs)-tRNAVal1-(2 base-pairs)-Su+ beta tRNALys-. In wild-type bacteria there are two identical tRNALys genes in one operon. Although we have shown that in Su+ beta it is the distal tRNALys that has been mutated to the ochre suppressor by a single base change at the anticodon (U36 to A36), we have not determined which of the two genes bears the supG or the supL mutation. The sequences following both tRNALys genes are highly homologous: both are about 100 base-pairs long and both terminate with an 18 base-pair sequence homologous to the last 18 bases of each tRNA. The sequences of tRNALys and tRNAVal1 are also very similar. Thus, including the 3'-portions of these tRNA genes, the 18 base-pair sequence is more or less periodically repeated five times in the DNA sequence.