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Involvement of Threonine Dehydratase in Biosynthesis of the -Ketobutyrate Prosthetic Group of Urocanase

¹ Department of Biochemistry, The Pennsylvania State University, University Park, Pennsylvania 16802

ABSTRACT

Seventeen mutants of Pseudomonas putida that were unable to grow on threonine as nitrogen source owing to a lack of threonine dehydratase were isolated, and all were found to be unable to synthesize active urocanase. Spontaneous revertants selected for urocanase production concomitantly regained threonine dehydratase. Mutants that were unable to utilize urocanate as carbon source were also isolated, and these were defective in urocanase formation but were normal in threonine dehydratase levels. Since -ketobutyrate is the prosthetic group for urocanase, these results are consistent with the proposal that threonine dehydratase is necessary for urocanase prosthetic group biosynthesis. However, the lack of urocanase activity in threonine dehydratase-negative mutants was shown not to be the result of reduced levels of endogenous free -ketobutyrate, nor to the participation of threonine dehydratase in the initiation of urocanase biosynthesis through the conversion of threonyl-tRNA^Thr to -ketobutyryl-tRNA^Thr. Other alternatives for the participation of threonine dehydratase in urocanase biosynthesis are discussed.