Alginate is the most important polysaccharide component that contributes to the cell wall of brown seaweeds or the extracellular matrix of some bacteria pathogens.Alginate lyases can degrade alginate via the β-elimination mechanism, producing oligosaccharides with unsaturated non-reducing (nr) ends.Therefore, alginate lyases are enzyme tools important for oligosaccharide preparation, energy fermentation, and medical treatment.Most bacteria within the genus Flammeovirga could efficiently degrade and grow on multiple polysaccharides, including alginate.However, few is known about the alginate-degrading systems.Herein we report AlgL-5, a 566-amino acid protein encoded by the genome of strain MY04, as the first alginate lyase found in Flammeovirga strains.The protein rAlgL-5 was an endo-type guluronate lyase.The recombinant enzyme rAlgL-5 showed the highest activities at 40 ℃ and pH 6.0 respectively.The smallest substrate was unsaturated pentosaccharide, while the minimal product was unsaturated disaccharide, yielded from the nr ends of 2-AB-labelled substrates.The final main alginate digestions contained series of unsaturated oligosaccharides, e.g.disaccharides, trisaccharides, and tetrasaccharides at a molar ratio of~ 1∶2.7∶2.8.Further 1H-spectrum analyses proved that AlgL-5 preferred to degrade alginate completely at G-rather than M-enriched motifs, thus producing oligosaccharides containing AG rather than AM as the nr end.Moreover, through gene truncation and enzyme characterization, we found that the non-catalytic region in AlgL-5 is essential to keep structural stabilities, biochemical characteristics, and substrate-degrading patterns of the enzyme.