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

Summary The gene of the amber suppressor tRNA derived from tRNA^Try, 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 of mature tRNA maintained by the gene is less than that of the normal haploid tRNA^Try 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 tRNA^Try, continues during extensive isoleucine limitation. The plasmid therefore contains a locus which probably alters the relaxedstringent circuit, whose effects is disseminated to at least 3 widely separated loci.