A cellulase/xylanase-negative mutant of Streptomyces lividans 1326 defective in cellobiose and xylobiose uptake is mutated in a gene encoding a protein homologous to ATP-binding proteins.

The uptake of monosaccharides (glucose and xylose) and disaccharides (cellobiose and xylobiose) was evaluated in the Streptomyces lividans mutant strain 10-164. The pleiotropic mutation had no effect on glucose uptake; however, the Vmax of xylose uptake was decreased 10-fold as compared to the wild-type strain, S. lividans 1326, and the transport system of cellobiose and xylobiose, the putative inducers of the cellulase and xylanase genes, was completely abolished resulting in a cellulase/xylanase-negative mutant. An accumulation of xylose and glucose in culture media was observed when the mutant was grown on xylobiose and cellobiose, respectively. Cell-associated beta-glucosidase and low levels of extracellular beta-glucosidase were detected in both strains. When gluconolactone, a beta-glucosidase inhibitor, was added to the medium there was no uptake of cellobiose or release of glucose by the mutant strain, whereas the uptake of cellobiose by the wild-type strain was not significantly affected. It is thus proposed that the active transport system for cellobiose and xylobiose is affected in mutant strain 10-164. Glucose and xylose production from disaccharide hydrolysis are due to beta-glucosidase and beta-xylosidase activities, which sustain the growth of the mutant strain. Clones complementing the mutation were isolated from a gene bank constructed using mutant strain 10-164. The msiK gene codes for MsiK, a 40 kDa multiple sugar import protein, which belongs to the family of ATP-binding proteins. The mutation is located in the B site which is responsible for ATP binding. This protein probably provides energy to the xylose and disaccharide transport system as a result of the hydrolysis of ATP.