l-Xylulose reductase (LXR) catalyzes the reduction of l-xylulose to xylitol in the fungal l-arabinose catabolic pathway. LXR (RpLXR) was purified from the pentose-fermenting zygomycetous fungus Rhizomucor pusillus NBRC 4578. The native RpLXR is a homotetramer composed of 29 kDa subunits and preferred NADPH as a coenzyme. The Km values were 8.71 mM for l-xylulose and 3.89 mM for dihydroxyacetone. The lxr3 (Rplxr3) gene encoding RpLXR consists of 792 bp and encodes a putative 263 amino acid protein (Mr = 28,341). The amino acid sequence of RpLXR showed high similarity to 3-oxoacyl-(acyl-carrier-protein) reductase. The Rplxr3 gene was expressed in Escherichia coli and the recombinant RpLXR exhibited properties similar to those of native RpLXR. Transcription of the Rplxr3 gene in R. pusillus NBRC 4578 was induced in the presence of l-arabinose and inhibited in the presence of d-glucose, d-xylose, and d-mannitol, indicating that RpLXR is involved in the l-arabinose catabolic pathway.
Keywords: Rhizomucor pusillus; l-Arabinose catabolism; l-xylulose reductase; zygomycetous fungus.