Arabinose-negative mutants of Escherichia coli B/r, ara-53 and ara-139, are deficient in the enzyme L-ribulose 5-phosphate 4-epimerase; ara-53, further analyzed, accumulates large quantities of L-ribulose 5-phosphate when incubated with L-arabinose. The mutant sites are closely linked to the left of the previously ordered L-arabinose mutant sites, and probably represent the structural gene for L-ribulose 5-phosphate 4epimerase (gene D) in the L-arabinose operon. The inducible levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase vary correspondingly as a result of mutation in the structural gene for L-ribulokinase (gene B), further substantiating the dual structural and regulatory function of this gene locus. Ara-53 and ara-139 are strongly inhibited by L-arabinose and give rise to L-arabinose-resistant mutants. The one resistant mutant analyzed still lacks the 4-epimerase but is deficient in L-ribulokinase and has increased L-arabinose isomerase activity, a characteristic of a type of mutation in the B gene. It is proposed that accumulation of Lribulose 5-phosphate is responsible for the inhibition, and that mutation to resistance will involve mutation in the A, B, C, permease, or repressor genes, thus providing a direct method for isolating these types of L-arabinose-negative mutants. Glucose prevents and cures the Larabinose inhibition.

Phosphodeoxyribomutase, the enzyme which catalyzes the interconversion of 2deoxyribose-l-phosphate to 2-deoxyribose-5-phosphate, has been partially purified from Salmonella typhimurium. The enzyme had an absolute requirement for manganese ion and was stimulated by glucose-1,6-diphosphate. Phosphodeoxyribomutase was induced by deoxyribose-5-phosphate and was coordinately regulated with the enzymes thymidine phosphorylase and deoxyribose-5-phosphate aldolase, type II. Mutants deficient in these three enzymes were isolated and mapped close to the threonine locus in S. typhimurium. The three enzymes thymidine phosphorylase, deoxyribose-5-phosphate aldolase, type II, and phosphodeoxyribomutase are controlled by a series of linked genes and appear to constitute an operon.

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