MUTATIONAL ANALYSIS OF THE NH2-TERMINAL ARMS OF THE TRP REPRESSOR INDICATES A MULTIFUNCTIONAL DOMAIN

The NH2-terminal arms of the Escherichia coli trp repressor have been implicated in three functions: formation of repressor-operator complex es via association with non-operator DNA; stabilization of repressor o ligomers bound to DNA; and oligomerization of the aporepressor in the absence of DNA. To begin to examine the structural aspects of the arms that are responsible for these varied activities, we generated an ext ensive set of deletion and substitution mutants and measured the activ ities of these mutants in vivo using reporter gene fusions. Deletion o f any part of the arms resulted in a significant decrease in repressor activity at both the trp and the trpR operons. Positions 4, 5 and 6 w ere the most sensitive to missense changes. Most substitutions at thes e positions resulted in repressors with less than 5% of the activity o f the wild-type trp repressor. A large percentage of the missense muta nts were more active than the wildtype repressor in medium containing tryptophan and less active in medium without tryptophan. This phenotyp e can be explained in terms of altered oligomerization of both the rep ressor and the aporepressor. Also, nine super-repressor mutants, resul ting from substitutions clustered at both ends of the arms, were found . Our results support the hypothesis that the NH2-terminal arm of the trp repressor is a multifunctional domain and reveal structural compon ents likely to be involved in the various functions.