An alginate lyase gene, algA, encoding a new poly beta-d-mannuronate (polyM)-specific alginate lyase AlgA, was cloned from Pseudomonas sp. E03. The recombinant AlgA with (His)(6)-tag, consisting of 364 amino acids (40.4 kDa),was purified using Ni-NTA Sepharose. The purified lyase had maximal activity (222 EU/mg) at pH 8 and 30 A degrees C and also maintained activity between pH 7-9 and below 45 A degrees C. It exclusively and endolytically depolymerized polyM by beta-elimination into oligosaccharides with degrees of polymerization (DP) of 2-5. Due to its high substrate specificity, AlgA could be a valuable tool for production of polyM oligosaccharides with low DP and for determining the fine structure of alginate.