Metabolism of L-rhamnose in Arthrobacter pyridinolis

In Arthrobacter pyridinolis, a respiration-coupled transport system for L-rhamnose caused accumulation of free L-rhamnose, while a phosphoenolpyruvate: L-rhamnose phosphotransferase system caused accumulation of L-rhamnose I-phosphate (Levinson & Krulwich, 1974). The pathways for subsequent metabolism of L-rhamnose and L-rhamose I-phosphate have now been investigated. Arthrobacter pyridinolis contains an inducible L-rhamnose isomerase and L-rhamnulokinase, as well as a constitutive L-rhamnulose I-phosphate aldolase. Results with mutants which are unable to metabolize L-rhamnose suggest the presence of an L-rhamnose I-phosphate phosphatase, which forms free L-rhamnose by hydrolysis of L-rhamnose I-phosphate produced by the phosphotransferase system. Mutants which lack this enzyme exhibited severe inhibition of growth in the presence of L-rhamnose plus any of a variety of carbon sources. There is some evidence that this inhibition was due to accumulation of L-rhamnose I-phosphate at toxic concentrations within the bacteria. The metabolism of L-rhamnose transported by the phosphotransferase system therefore appears to occur by hydrolysis of L-rhamnose I-phosphate to free L-rhamnose by a phosphatase. Metabolism of the L-rhamnose thus produced, and of that accumulated by the respiration-coupled transport system, the proceeds by the sequence of reactions: L-rhamnose leads to L-rhamnulose leads to L=rhamnulose I-phosphate leads to dihydroxyacetone phosphate plus L-lactaldehyde.