SUMMARY: synthesizes one major tryptophan transfer ribonucleic acid (tRNA) species (isoacceptor A) carrying characteristic base modifications. Under tryptophan limited growth conditions wild-type strain X2180-1A exhibited a second important tRNA species (isoacceptor B). The total amount of tRNA, approximately 2.5 pmol (mg dry wt), stayed essentially constant during amino acid shift-down experiments. The amount of isoacceptor B relative to total tRNA was 10 to 15% during amino acid sufficient exponential growth conditions, but increased during tryptophan limitation three- to four-fold. Analysis of the base compositions showed that isoacceptor B differed from isoacceptor A in one respect only: 2′--methylguanosine, a modified guanosine base occurring at position 17 of the major isoacceptor A tRNA, was not detectable in hydrolysates of purified isoacceptor B. The biological significance of isoacceptor B is discussed.


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