Isolation and properties of a plasmid which expresses the E. coli Su+7 amber suppressor tRNA gene

The gene of the amber suppressor tRNA derived from tRNATry, Su+7, has been inserted into a col E1-derived vehicle by selecting for its expression. Despite selection for a suppressor phenotype, and the plasmid's stable presence at ca. 180 copies cell during balanced growth, the level mature tRNA maintained by the gene is less than that of the normal haploid tRNATry locus in the bacterial chromosome. Transfer RNA genes, both the plasmid Su+7 gene and chromosomal tRNA's are expressed during inhibition of protein synthesis. During, e.g. chloramphenicol inhibition, Su-7 and Su+7 tRNA can be elevated similarly in the plasmid-containing cell; Su+7 reaches levels of molecules/cell which ordinarily characterize a major tRNA. The recombinant plasmid, but not the cloning vehicle alone, has a more general effect on tRNA levels; accumulation of tRNA from three chromosomal tRNA loci including tRNATry, continues during extensive isoleucine limitation. The plasmid therefore contains a locus which probably alters the relaxed-stringent circuit, whose effect is disseminated to at least 3 widely separated loci.