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The capacity to both ferment and oxidize l -ascorbate has been widely documented for a number of enteric bacteria. Here we present evidence that all the strains of Klebsiella pneumoniae tested in this study ferment l -ascorbate using the ula regulon-encoded proteins. Under aerobic conditions, several phenotypes were observed for the strains. Our results showed that the yiaK-S system is required for this aerobic metabolic process. Gel shift experiments performed with UlaR and YiaJ and probes corresponding to the specific promoters indicated that l -ascorbate-6-phosphate is the effector molecule recognized by both regulators, since binding of the repressors to their recognition sites was impaired by the presence of this compound. We demonstrated that in K. pneumoniae cells l -ascorbate-6-phosphate is formed only by the action of the UlaABC phosphotransferase system. This finding explains why strains that lack the ula genetic system and therefore are unable to form the inducer intracellularly cannot efficiently use this vitamin as a carbon source under either anaerobic or aerobic conditions. Thus, efficient aerobic metabolism of l -ascorbate in K. pneumoniae is dependent on the presence of both the yiaK-S and ula systems. The expression of the yiaK-S operon, but not the expression of the ula regulon, is controlled by oxygen availability. Both systems are regulated by the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex and by IHF.

Volume 89, no. 2, p. [1105–1118][1], 2015. Page 1116, Acknowledgments: The following sentence should appear at the end of the second paragraph. A portion of this work was supported by an American Foundation for AIDS Research Mathilde Krim Fellowship in Basic Biomedical Research (L.K.). [1]: /