ACTIVATION OF A CRYPTIC GENE ENCODING A KINASE FOR L-XYLULOSE OPENS ANEW PATHWAY FOR THE UTILIZATION OF L-LYXOSE BY ESCHERICHIA-COLI

A silent gene encoding a kinase that specifically phosphorylates L-xyl ulose was activated and rendered constitutive in mutant cells of Esche richia coli. L-Xylulose kinase was purified to homogeneity and found t o be a dimer of two subunits of 55 kDa, highly specific for L-xylulose with a k(m) of 0.8 mM, a V-max of 33 mu mol/min/mg, and an optimum pH of 8.4. Physical (thin layer chramatography) and spectroscopic (nucle ar magnetic resonance and optical rotation) characterization of the pr oduct of L-xylulose kinase indicated that the enzyme phosphorylated th e sugar at position 5. The gene encoding L-xylulose kinase was mapped in the 80.2 min region of the chromosome by conjugation and transducti on. Cloning and comparison of the restriction map with the Kohara map (Kohara, Y., Akiyame, K., and Isono, K. (1987) Cell 50, 495-501) locat ed the gene between positions 3963 and 3965 kilobases. The molecular a nd functional features of L-xylulose kinase together with the location of the corresponding gene indicate that this enzyme did not derive fr om mutation of any other known kinase. The new kinase opens a route fo r the utilization of L-lyxose through the action of rhamnose permease, rhamnose isomerase, and the phosphorylation of the L-xylulose formed to L-xylulose 5-phosphate, which is then introduced into the pentose p hosphate pathway for subsequent metabolism.