Cloning of an alkaline phosphatase gene from the moderately thermophilic bacterium Meiothermus ruber and characterization of the recombinant enzyme

A gene that codes for an alkaline phosphatase was cloned from the thermophilic bacterium Meiothermus ruber, and its nucleotide sequence was determined. The deduced amino acid sequence indicates that the enzyme precursor including the putative signal sequence is composed of 503 amino acid residues and has an estimated molecular mass of 54,229 Da. Comparison of the peptide sequence with that of the prototype alkaline phosphatase from Escherichia coli revealed conservation of the regions in the vicinity of the corresponding phosphorylation site and metal binding sites. The protein was expressed in E. coli and its enzymatic properties were characterized. In the absence of exogenously added metal ions, activity was negligible; to obtain maximal activity, addition of free Mg2+ ions was required. Zn2+ ions had an inhibitory effect on the activity of the M. ruber enzyme. The pH and temperature optima for activity were found to be 11.0 and 62°C, respectively. The enzyme was moderately thermostable: it retained about 50% activity after incubation for 6 h at 60°C, whereas at 80°C it was completely inactivated within 2 h. The Michaelis constant for cleavage of 4-nitrophenylphosphate was 0.055 mM. While having much in common with other alkaline phosphatases, the M. ruber enzyme presents some unique features, such as a very narrow pH range for activity and an absolute requirement for magnesium for activity.