The molecular basis for the differential translation of TMV RNA in tobacco protoplasts and wheat germ extracts.

Translation of tobacco mosaic virus (TMV) RNA in tobacco protoplasts yields the 17.5-K coat protein, a 126-K protein and a 183-K protein which is generated by an efficient readthrough over the UAG termination codon at the end of the 126-K cistron. In wheat germ extracts, however, only the 5'-proximal 126-K cistron is translated whereas the 183-K readthrough protein is not synthesized. Purification and sequence analysis of the endogenous tyrosine tRNAs revealed that the uninfected tobacco plant contains two tRNAs, both with GPsiA anticodons which stimulate the UAG readthrough in vitro and presumably in vivo. In contrast, 85% of the tRNA from wheat germ contains a QPsiA anticodon and 15% has a GPsiA anticodon. Otherwise the sequences of tRNAs from wheat germ and tobacco are identical. UAG readthrough and hence synthesis of the 183-K protein is only stimulated by tRNA(GPsiA) and not at all by tRNA(QPsiA). The tRNAs from wheat leaves were also sequenced. This revealed that adult wheat contains tRNA(GPsiA) only. This is very much in contrast to the situation in animals, where Q-containing tRNAs are characteristic for adult tissues whereas Q deficiency is typical for the neoplastic and embryonic state.