Galactose metabolism in Rhizobiaceae. Characterization of exoB mutants

Agvobactevium tumefaciens exoB mutants (deficient in succinoglycan or EPS I synthesis) were characterized. The defect was determined to be the lack of UDP-glucose 4-epimerase (EC 5.1.3.2) activity. This defect results in absence of UDP-galactose, which is the obligatory intermediate as galactose donor for initiation of the succinoglycan repeating unit on a lipid intermediate. It was determined that A. tumefaciens does not have the Leloir pathway enzyme activities galactokinase (EC 2.7.1.6) and galactose-1-phosphate uridylyltransferase (EC 2.7.7.10). A De Ley-Doudoroff oxidative pathway for galactose metabolism was observed in A. tumefaciens. This pathway is not constitutive and its expression required D-galactose or L-arabinose. However, UDP-glucose 4-epimerase is constitutive and did not require growth on galactose for full expression