The Bacillus stearothermophilus NUB36 surA gene encodes a thermophilic sucrase related to Bacillus subtilis SacA.

The complete nucleotide sequence of the surA gene, encoding a sucrase from Bacillus stearothermophilus NUB36, was determined. surA was composed of 1338 bp and encoded 445 amino acid residues. The deduced polypeptide of M(r) 51519 showed strong sequence similarity to sucrose and sucrose phosphate hydrolases from Bacillus subtilis, Klebsiella pneumoniae and Vibrio alginolyticus, and contained the 'sucrose box' residues thought to be important for catalysis of the transfer of fructose from sucrose. The enzyme was partially purified using affinity chromotography from extracts of Escherichia coli containing the cloned surA. SurA displayed an optimum temperature for sucrose hydrolysis of 55 degrees C and high stability. The M(r) of SurA determined by gel filtration was 105,000, which suggested that the active form of the enzyme is a dimer. SurA exhibited an apparent Km of 40 mM for sucrose but, unlike the homologous B. subtilis enzyme, had no detectable sucrose phosphate hydrolase activity.