Tryptophan operon read-through. Isolation and characterization of an abnormally long tryptophan synthetase alpha subunit from a frame-shift mutant of Escherichia coli

A new mutant strain of Escherichia coli, strain ICR-47, contains a frame-shift mutation in the trpA gene, the gene most distal to the operator in the trp operon. Mapping experiments indicate that the lesion is located at a site within 10 to 15% of the end of this gene. The mutation results in "out-of-phase" translation of the distal portion of the trp mRNA; normal translational termination signal(s) are not encountered and a trpA gene product longer than the wild type protein is produced. As with the other enzymes produced from this operon, the in vivo level of the altered protein (the alpha subunit of the tryptophan synthetase enzyme complex) is controlled by exogenous L-tryptophan. The altered alpha subunit from the strain ICR-47 has been isolated and characterized. Molecular weight estimations indicate a molecular weight of approximately 37,000, an increase beyond the wild type enzyme corresponding to an additional 50 to 70 amino acid residues. The protein has a new COOH-terminal amino acid sequence. Results of preliminary hybridization experiments suggest that the ICR-47 mRNA, which is necessarily longer than that needed to code for wild type enzyme, is not detectably different in size from wild type mRNA. The enzymatic properties of the ICR-47 alpha subunit indicates a greatly reduced ability of the mutant subunit to combine functionally with wild type beta2 subunit, the second protein component in the tryptophan synthetase enzyme complex. In contrast, only 40 to 50% of the intrinsic enzymatic activity of the alpha subunit is lost.