Portal de Periódicos da CAPES

Nonsense codons and polarity in the tryptophan operon

1966; Elsevier BV; Volume: 21; Issue: 2 Linguagem: Inglês

10.1016/0022-2836(66)90102-1

1089-8638

Charles Yanofsky, Junetsu Ito,

Polyamine Metabolism and Applications

Amber and ochre mutants and mutants with cross-reacting material for all five genes of the tryptophan operon have been isolated and characterized. Co-ordinate enzyme formation was examined under repression conditions with a repressordeficient stock of each mutant. It was found that alterations resulting in the production of cross-reacting material had little or no effect on relative rates of synthesis of tryptophan enzymes, whereas amber and ochre nonsense alterations invariably had polar effects—they reduced the relative rates of synthesis of all proteins specified by genes in the operon more distal from the operator region than the gene with the nonsense mutation. Nonsense alterations on the side of a gene close to the operator region had a more pronounced effect on the synthesis of proteins specified by more distal genes in the operon than alterations near the other end of the gene. Amber and ochre alterations had equivalent effects on enzyme formation. Represser-deficient stocks were prepared with nonsense alterations in two different genes in the operon. Each nonsense alteration acted independently in reducing the relative rate of synthesis of the protein specified by the last gene of the operon. Repressor-deficient stocks with two nonsense alterations in the same gene behaved in co-ordinate enzyme production studies as if only one nonsense alteration were present. Suppressor genes relieved the polar effect, approximately to the same extent as they permitted synthesis of the protein specified by the gene with the nonsense alteration. Under de-repression conditions, nonsense alterations in the first gene of the operon had the most profound effect on enzyme formation. An interpretation of these findings is presented which suggests that ribosomes can only become attached at the operator end of the polycistronic tryptophan messenger and that ribosomes leave the messenger in regions between an introduced nonsense codonand the start of the next message. Polarity would result from the reduction of the number of active ribosomes on the messenger following the encounter with the nonsense codon and the subsequent untranslatable region.