All of the alginate lyase species (A1-I, A1-II and A1-III) in a bacterium (strain Al) isolated from a ditch are generated from a common precursor protein (A1-0) having a molecular size of 69 kDa (Murata et al., J. Ferment. Bioeng., 76, 427-437, 1993). A1-I (63 kDa) generated after removal of an N-terminal peptide (6 kDa) from A1-0 was found to be converted into A1-II (23 kDa) and A1-III (40 kDa) by a self-processing reaction of the A1-I molecule itself.