Genes for the alpha and beta subunits of the phenylalanyl-transfer ribonucleic acid synthetase of Escherichia coli

Abstract

The phenylalanyl-tRNA synthetase of E. coli is a tetramer that contains 2 different kinds of polypeptide chains. To locate the genes for the 2 polypeptides, temperature-sensitive mutants with defective phenylalanyl-tRNA synthetases were analyzed to see which subunit was altered. The method was in vitro complementation; mutant cell extracts were mixed with purified separated .alpha. or .beta. subunits of the wild-type enzyme to generate an active hybrid enzyme. With 3 mutants, enzyme activity appeared when .alpha. was added, but not when .beta. was added: these are therefore assumed to carry lesions in the gene for the .alpha. subunit. Two other mutants gave the opposite response and are presumably .beta. mutants. Enzyme activity is also generated when .alpha. and .beta. mutant extracts are mixed, but not when 2 .alpha. or 2 .beta. mutant extracts are mixed. The inactive mutant enzymes may be dissociated, as judged by their sedimentation in sucrose density gradients, but the dissociation may be only partial. The active enzyme generated by complementation occurred in 2 forms, 1 that resembled the native wild-type enzyme and 1 that sedimented more slowly. Both .alpha. and .beta. mutants are capable of generating the native form, although .alpha. mutants require prior urea denaturation of the defective enzyme. With the mutants thus characterized, the genes for the .alpha. and .beta. subunits (designated pheS and pheT, respectively) were mapped. The gene order, as determined by transduction, is aroD-pps-pheT-pheS. The pheS and pheT genes are close together and may be immediately adjacent.