ABSTRACT
Tyrosine phenol-lyase (Tpl), which can synthesize 3,4-dihydroxyphenylalanine from pyruvate, ammonia, and catechol, is a tyrosine-inducible enzyme. Previous studies demonstrated that thetpl promoter of Erwinia herbicola is activated by the TyrR protein of Escherichia coli. In an attempt to create a high-Tpl-expressing strain, we cloned the tyrRgene of E. herbicola and then randomly mutagenized it. Mutant TyrR proteins with enhanced ability to activate tplwere screened for by use of the lac reporter system inE. coli. The most increased transcription oftpl was observed for the strain with the mutanttyrR allele involving amino acid substitutions of alanine, cysteine, and glycine for valine-67, tyrosine-72, and glutamate-201, respectively. A tyrR-deficient derivative of E. herbicola was constructed and transformed with a plasmid carrying the mutant tyrR allele (V67A Y72C E201G substitutions). The resultant strain expressed Tpl without the addition of tyrosine to the medium and produced as much of it as was produced by the wild-type strain grown under tyrosine-induced conditions. The regulatory properties of the mutant TyrRV67A, TyrRY72C, TyrRE201G, and TyrRV67A Y72C E201G proteins were examined in vivo. Interestingly, as opposed to the wild-type TyrR protein, the mutant TyrRV67A protein had a repressive effect on the tyrP promoter in the presence of phenylalanine as the coeffector.
Insights into the Catalytic Mechanism of Tyrosine Phenol-lyase from X-ray Structures of Quinonoid Intermediates
