The gene coding for ribose-5-phosphate isomerase (Rpi) from Thermotoga lettingae TMO was cloned and expressed in E. coli. The recombinant enzyme was purified by Ni-affinity chromatography. It converted d-psicose to d-allose maximally at 75 A degrees C and pH 8.0 with a 32 % conversion yield. The k (m), turnover number (k (cat)), and catalytic efficiency (k (cat) k (m) (-1) ) for substrate d-psicose were 64 mM, 6.98 min(-1) and 0.11 mM(-1) min(-1) respectively.