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Abstract
SUMMARY: 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 l-phosphate ( Levinson & Krulwich, 1974 ). The pathways for subsequent metabolism of l-rhamnose and l-rhamnose 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, then proceeds by the sequence of reactions: l-rhamnose →l-rhamnulose → l-rhamnulose i-phosphate → dihydroxyacetone phosphate plus l-lactaldehyde.
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