Escherichia coli has the potential to be a powerful biocatalyst for the conversion of lignocellulosic biomass into useful materials such as biofuels and polymers. One important challenge in using E. coli for the transformation of biomass sugars is diauxie, or sequential utilization of different types of sugars. We demonstrate that, by increasing the intracellular levels of the transcription factor XylR, the preferential consumption of arabinose before xylose can be eliminated. In addition, XylR augmentation must be finely tuned for robust coutilization of these two hemicellulosic sugars. Using a novel technique for scarless gene insertion, an additional copy of xylR was inserted into the araBAD operon. The resulting strain was superior at cometabolizing mixtures of arabinose and xylose and was able to produce at least 36% more ethanol than wild-type strains. This strain is a useful starting point for the development of an E. coli biocatalyst that can simultaneously convert all biomass sugars.