Structural and functional analyses of alginate lyases are important in the clarification of the biofilm-dependent ecosystem in P. aeruginosa, and the development of therapeutic agents for bacterial disease. Most alginate lyases are classified into polysaccharide lyase (PL) family-5 and -7 based on their primary structures. Family PL-7 enzymes are still poorly characterized especially in structural properties. Among family PL-7, a gene coding for a hypothetical protein (PA1167) homologous to Sphingomonas alginate lyase A1-II was found to be present in the P. aeruginosa genome. PA1167 overexpressed in Escherichia coli cleaved glycosidic bonds in alginate and released unsaturated saccharides, indicating that PA1167 is an alginate lyase catalyzing a -elimination reaction. The enzyme acted preferably on heteropolymeric regions endolytically, and worked most efficiently at pH 8.5 and 40. The specific activity of PA1167, however, was much weaker than that of the known alginate lyase AlgL, suggesting that AlgL plays a main role in alginate depolymerization in P. aeruginosa. In addition to this specific activity, differences were found between PA1167 and AlgL in enzyme properties such as molecular mass, optimum pH, salt effect, and substrate specificity. The first crystal structure of the family PL-7 alginate lyase was determined at 2.0 resolution. PA1167 was found to form a glove-like -sandwich composed of 15 -strands and 3 -helices. The structural difference between the -sandwich PA1167of family PL-7 and /-barrel AlgL of family PL-5 may be responsible for the enzyme characteristics. Crystal structures of polysaccharide lyases so far determined indicate that they can be assigned to 3 folding groups having parallel -helix, /-barrel, and /-barrel + antiparallel -sheet structures as basic frames. PA1167 is the fourth novel folding structure found among polysaccharide lyases.