(A) novel D-allose operon in escherichia coli K-12 and its secondary function as a D-ribose transporter

Abstract

A search for a gene responsible for a low-affinity transporter of D-ribose revealed that the Escherichia coli K-12 strain MC4100 utilizes D-allose as a sole carbon source by having D-allose transporter. The operon involved in D-allose metabolism was localized at 92.8 min of the E. coli linkage map. It consists of six genes which are inducible by D-allose and are under the control of the repressor gene alsR. This operon is also subject to catabolite repression. Three genes, alsB, alsA, and alsC, appear to be necessary for transport of D-allose as well as for transport of structurally related D-ribose at low affinity. D-allose-binding protein encoded by alsB is a periplasmic protein, which has an affinity to D-allose with a Kd of 0.33 micro M. The transport system has a feature of binding protein-mediated ABC-transporter which includes an ATP-binding component (AlsA). It was found that AlsE (a putative D-allulose-6-phosphate 3-epimerase), neither AlsK (a putative D-allose kinase) nor RpiB (a putative D-allose-6-phosphate isomerase), is necessary for allose metabolism. During this study, the strain W3110, a well-known wild type E. coli, was found to have a defect in the transport of D-allose mediated by the allose permease. It was observed that Escherichia coli can still grow on D-ribose in the absence of both the ribose and the allose permease. Futhermore, since several partially characterized mutations also affects the ribose growth (Rha-, xylA-, a locus linked to rbs operon), it seems that low-affinity D-ribose transport is mediated by a sugar specific system such as in the cases of D-xylose and L-arabinose which have their specific low-affinity transport systems.