Utilization of l -Ascorbate by Escherichia coli K-12: Assignments of Functions to Products of the yjf-sga and yia-sgb Operons

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Abstract
Escherichia coli K-12 can ferment l -ascorbate. The operon encoding catabolic enzymes in the utilization of l -ascorbate ( ula ) has been identified; this operon of previously unknown function had been designated the yif-sga operon. Three enzymes in the pathway that produce d -xylulose 5-phosphate have been functionally characterized: 3-keto- l -gulonate 6-phosphate decarboxylase (UlaD), l -xylulose 5-phosphate 3-epimerase (UlaE), and l -ribulose 5-phosphate 4-epimerase (UlaF). Several products of the yia-sgb operon were also functionally characterized, although the substrate and physiological function of the operon remain unknown: 2,3-diketo- l -gulonate reductase (YiaK), 3-keto- l -gulonate kinase (LyxK), 3-keto- l -gulonate 6-phosphate decarboxylase (SgbH), and l -ribulose 5-phosphate 4-epimerase (SgbE).