The effect of an Escherichia coli regulatory mutation on transfer RNA structure

Abstract

The trpX mutation in Escherichia coli reduces trp operon attenuation in strains carrying wild-type tRNA Trp. The trpX − phenotype is alleviated (attenuation is restored) in UGA-suppressor tRNA Trp-carrying strains (Yanofsky & Soll, 1977). The tRNA from various trpX − strains was characterized biochemically. Sequence analyses of wild-type tRNA Trp and UGA suppressor tRNA Trp, both derived from trpX − strains, reveal an unmodified A in the position (adjacent to the anticodon) normally occupied by the hypermodified base ms 2i 6A. In addition, several tRNAs from trpX − cells were characterized by RPC-5 column chromatography. We find that only tRNAs normally having ms 2i 6A exhibit altered elution profiles when compared to the homologous tRNAs from trpX − cells. Introduction of the UGA suppressor into trpX − cells does not restore normal Chromatographic behavior. These results suggest that the trpX gene product is necessary for the synthesis of ms 2i 6A. Thus, we propose that miaA (for the first gene involved in ms 2 i 6 A synthesis) replaces the trpX designation. The results reported here are discussed with regard to a model proposed by Lee & Yanofsky (1977) in which efficient translation of the tandem trp codons in the leader sequence RNA is required for normal attenuation of the trp operon.