Cloning of the Escherichia coli sor genes for L-sorbose transport and metabolism and physical mapping of the genes near metH and iclR.

The sor genes for L-sorbose (Sor) degradation of Escherichia coli EC3132, a wild-type strain, have been cloned on a 10.8-kbp fragment together with parts of the metH gene. The genes were mapped by restriction analysis, by deletion mapping, and by insertion mutagenesis with Tn1725. Seven sor genes with their corresponding gene products have been identified. They form an operon (gene order sorCpCDFBAME) inducible by L-sorbose, and their products have the following functions: SorC (36 kDa), regulatory protein with repressor-activator functions; SorD (29 kDa), D-glucitol-6-phosphate dehydrogenase; SorF and SorB (14 and 19 kDa, respectively), and SorA and SorM (27 and 29 kDa, respectively), two soluble and two membrane-bound proteins, respectively, of an L-sorbose phosphotransferase transport system; SorE (45 kDa), sorbose-1-phosphate reductase. The sor operon from E. coli EC3132 thus is identical to the operon from Klebsiella pneumoniae KAY2026. On the basis of restriction mapping followed by Southern hybridization experiments, the sor genes were mapped at 91.2 min on the chromosome, 3.3 kbp downstream of the metH-iclR gene cluster, and shown to be transcribed in a counterclockwise direction. The chromosomal map of the Sor+ strain EC3132 differs from that of the Sor- strain K-12 in approximately 8.6 kbp.