ANALYSIS OF HETERODIMER FORMATION BY THE ESCHERICHIA-COLI TRP REPRESSOR

The trp repressor of Escherichia coli is a dimeric DNA-binding protein that regulates transcription of several operons concerned with trypto phan metabolism. Although heterodimer formation between mutant and wil d type subunits occurs readily in vivo, comparable heterodimers could be formed in vitro only under extreme conditions. To explain this diff erence we analyzed trp repressor dimer formation and dissociation usin g an in vitro transcription/translation system. Nascent wild type or m utant repressor polypeptides, synthesized in the presence of an excess of a second repressor, were invariably incorporated into heterodimers . In contrast, previously synthesized and assembled wild type dimers a ppeared to be refractory to dissociation, since they did not form hete rodimers. However, previously synthesized mutant dimeric repressors th at were defective in tryptophan binding readily dissociated and formed heterodimers. We noted that the ability of a dimeric repressor to dis sociate under our conditions correlated inversely with its affinity fo r tryptophan. Consistent with this conclusion, we found that dissociat ion of the wild type aporepressor (no added tryptophan) was appreciabl y more rapid than dissociation of the tryptophan-saturated wild type r epressor.