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 compo sed of 1338 bp and encoded 445 amino acid residues. The deduced polype ptide of M(r) 51 519 showed strong sequence similarity to sucrose and sucrose phosphate hydrolases from Bacillus subtilis, Klebsiella pneumo niae and Vibrio alginolyticus, and contained the 'sucrose box' residue s thought to be important for catalysis of the transfer of fructose fr om sucrose. The enzyme was partially purified using affinity chromotog raphy from extracts of Escherichia coli containing the cloned surA. Su rA displayed an optimum temperature for sucrose hydrolysis of 55 degre es 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 K-m of 40 mM for sucrose but, unlike the homologous B. subtilis enzyme, had no detectable sucrose phosphat e hydrolase activity.