Regulation of the l-arabinose catabolic pathway in Aerobacter aerogenes

Abstract

Three enzymes of the l-arabinose catabolic pathway in Aerobacter aerogenes, l-arabinose isomerase, l-ribulokinase, and l-ribulose-5-phosphate 4-epimerase, are specifically induced in the presence of l-arabinose. Mutants constitutive for kinase activity are also constitutive for the isomerase and 4-epimerase activities, suggesting that these three enzymes are coordinately controlled in A. aerogenes. l-Ribulokinase activity can still be induced in the presence of l-arabinose in an isomerase-deficient strain of A. aerogenes. Since l-arabinose is not converted to l-ribulose in such a strain, it appears that l-arabinose must be the inducer of l-ribulokinase, as well as the coordinately controlled isomerase and 4-epimerase. As in the metabolism of l-arabinose, growth of A. aerogenes on l-arabitol also requires a 4-epimerase for the conversion of l-ribulose-5-phosphate to d-xylulose-5-phosphate. However, loss of ability to metabolize l-arabinose, due to a deficiency in 4-epimerase synthesis in the presence of l-arabinose, does not affect growth on l-arabitol. In addition, synthesis of the 4-epimerase associated with l-arabitol metabolism is not accompanied by l-arabinose isomerase or l-ribulokinase synthesis. These results suggest either the existence of two different l-ribulose-5-phosphate 4-epimerases in A. aerogenes, or of two different regulatory mechanisms for the control of the same epimerase.