A newly isolated bacterium, Cellulomonas parahominis MB426, produced L-ribose isomerase (CeLRI) on a medium containing L-ribose as a sole carbon source. A 32 kDa protein isomerizing L.-ribose to L-ribulose was purified to homogeneity from this bacterium. A set of degenerated primers were synthesized based on amino acid sequences of the purified CeLRI, and a 747 bp gene encoding CeLRI was cloned, sequenced and overexpressed in Escherichia colt. This gene encoded a 249 amino acid protein with a calculated molecular mass of 27,435. The deduced amino acid sequence of this gene showed the highest identity with L-ribose isomerase from Acinetobacter calcoaceticus DL-28 (71%). The recombinant L-ribose isomerase (rCeLRI) was optimally active at pH 9.0 and 40 degrees C, and was stable up to 40 degrees C for 1 h and not dependent for metallic ions for its activity. The rCeLRI showed widely substrate specificity for the rare sugar which involved L-erythro form such as L-ribose, D-lyxose, D-talose, D-mannose, L-gulose, and L-allose. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.