SYNTHETASE A-PROTEIN MUTANT A36*

Various mutants with changes in the gene specifying the structure of the A protein of tryptophan synthetase from Escherichia colt can be suppressed by mutations in other genes. 1 Mutant strains carrying such active suppressor genes produce two structurally distinct A proteins, one immunologically indistinguishable from normal A protein but nonfunctional (designated A-CRM), and the other, functional and apparently normal in all respects.2' This type of suppression is thought to result from mistakes in the translation process, i.e., two amino acids are specified by the same codon. 1, I For example, the A protein produced by the mutant A36 contains an arginine residue at a position normally occupied by glycine. In the suppressed mutant (A36 su36+), two A proteins (separable by DEAE-cellulose chromatography), one with arginine and the other with glycine in the same position, are produced.3 From reversion studies with mutant A36, and analyses of the amino acids which can replace arginine at the mutant site, the most likely codon change resulting from the A36 mutation is GGA(gly) -* AGA(arg).5 Recently, it was shown that the alternating copolymer, poly (AG), synthesized with RNA polymerase and poly d(AG: TC), directs the synthesis of an alternating copolypeptide of arginine and glutamic acid in an in vstro amnino acid-incorporating system.6 It seemed possible that this system could be used to study the mechanism of suppression of the AGA codon, i.e., could glycine be incorporated in place of arginine in response to poly (AG) tn vstro? We have found that a ribosome-soluble protein system, when supplemented with poly (AG) and tRNA isolated from the Su36 + strain, can incorporate glycine into an alternating sequence with glutamic acid. Glycine incorporation does not occur when the identical system is supplemented with tRNA from cells lacking this suppressor. In an accompanying paper,7 Gupta et al. have studied a suppressor of the