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Glucose is known to inhibit the transport and metabolism of many sugars inEscherichia coli . This mechanism leads to its preferential consumption. Far less is known about the preferential utilization of nonglucose sugars inE. coli . Two exceptions arel -arabinose andd -xylose. Previous studies have shown thatl -arabinose inhibitsd -xylose metabolism inEscherichia coli . This repression results froml -arabinose-bound AraC binding to the promoter of thed -xylose metabolic genes and inhibiting their expression. This mechanism, however, has not been explored in single cells. Both thel -arabinose andd -xylose utilization systems are known to exhibit a bimodal induction response to their cognate sugar, where mixed populations of cells either expressing the metabolic genes or not are observed at intermediate sugar concentrations. This suggests thatl -arabinose can only inhibitd -xylose metabolism inl -arabinose-induced cells. To understand how cross talk between these systems affects their response, we investigatedE. coli during growth on mixtures ofl -arabinose andd -xylose at single-cell resolution. Our results showed that mixed, multimodal populations ofl -arabinose- andd -xylose-induced cells occurred at intermediate sugar concentrations. We also found thatd -xylose inhibited the expression of thel -arabinose metabolic genes and that this repression was due to XylR. These results demonstrate that a strict hierarchy does not exist betweenl -arabinose andd -xylose as previously thought. The results may also aid in the design ofE. coli strains capable of simultaneous sugar consumption.IMPORTANCE Glucose,d -xylose, andl -arabinose are the most abundant sugars in plant biomass. Developing efficient fermentation processes that convert these sugars into chemicals and fuels will require strains capable of coutilizing these sugars. Glucose has long been known to repress the expression of thel -arabinose andd -xylose metabolic genes inEscherichia coli . Recent studies found thatl -arabinose also represses the expression of thed -xylose metabolic genes. In the present study, we found thatd -xylose also represses the expression of thel -arabinose metabolic genes, leading to mixed populations of cells capable of utilizingl -arabinose andd -xylose. These results further our understanding of mixed-sugar utilization and may aid in strain design.

Reciprocal Regulation of l -Arabinose and d -Xylose Metabolism in Escherichia coli