Function and regulation of D-xylose operons in escherichia coli K-12

Abstract

Mutants defective in the high-affinity transport for D-ribose are still able to utilize the sugar as a sole carbon and energy source, suggesting that other low-affinity transport systems for the sugar exist in the cells. In order to search for these transport systems, transposon mutagenesis for the high affinity transporter-defective cell was performed and several dozens of candidates showing different phenotypes on D-ribose minimal growth were isolated with a frequency of $1 \times 10^{-5}$. The precise locations of insertions on the chromosome were determined by in vivo cloning and analysis using polA mutation. By genetic and biochemical studies of the mutants it was revealed that two transport systems for D-xylose (XylFGH) and D-allose (AlsBAC) are involved in the uptake of D-ribose into cells with low affinity. In case of Als system, its contribution to D-ribose growth is elevated by a point mutation in the negative regulator alsR. Atypical insertions showing an enhanced D-ribose growth were appeared and mapped at xylA and its promoter region. The mutations elevated not only the uptake rate for D-ribose but also the expression of xylFGH encoding high-affinity transporter for D-xylose. Suppressor mutations resulting in the basal growth were obtained in xylF, xylG, and xylR. The elevated level of D-ribose uptake was lowered to basal level both in the presence of D-xylose and by the suppressor mutations. Also, a constitutive expression of xylFGH under $P_{trc}$ promoter enhanced the growth for D-ribose. These results indicate that XylFGH transports D-ribose into cell. The metabolism of D-xylose in Escherichia coli K-12 was known to be mediated by xylAB gene. However, the xylFGH and xylR genes were recently found by genome sequencing and predicted for transport and regulation based on their sequence similarities. The functions and regulation of the xyl genes were also investigated. An analysis with transposon insertions in xyl genes revealed that the xyl genes ...