Two distinct types of mutations conferring to Escherichia coli K12 capability of D-tryptophan utilization.

We showed that the ability of Escherichia coli K12 tryptophan auxotrophs to utilize D-tryptophan as a substitute for L-tryptophan may result from two types of mutations. The first type consisted in changes in the dadR regulatory site of the dad operon increasing the synthesis of D-amino acid dehydrogenase. The mutations of the second type mapped within the dad A structural gene. They changed the apparent substrate specificity of D-amino acid dehydrogenase. We suppose that the change may be due to an altered enzyme structure which make it more accessible to D-tryptophan.