Abstract: The invention concerns identification of a gene encoding a novel L-arabinose isomerase of the Bacillus stearothernivphilus strain US 100 (L-AI US 100), a L-arabinose isomerase expressed from said gene, recombinant vectors harbouring said gene, microorganisms transformed with said vector, a protocol for preparing and purifying said recombinant protein, biochemical and kinetic characterization of said recombinant enzyme and a method for bioconversion of a D-galactose solution into a solution rich in D-tagatose using said polypeptide. This novel protein has original characteristics, in particular its independence from metal ions for its activity and its low need for such ions for its thermostability, as well as its potential for isomerizing D-galactose into D-tagatose with great efficacy of about 48% after 7 hours at 70° C.