Plays two roles in the biosynthesis of the exopolysaccharide alginate: protects alginate from degradation as the polymer traverses the periplasm, and also plays a role in its O-acetylation. Acetylation of alginate causes the cells in the biofilm to adhere better to lung epithelium, form microcolonies, and resist the effects of the host immune system and/or antibiotics. Displays a low acetylesterase activity in vitro using a pseudosubstrate, 3-carboxyumbelliferyl acetate. Probably has acetyltransferase activity in vivo. Glycan biosynthesis; alginate biosynthesis. Monomer. Interacts with AlgK and MucD. Consists of two domains, with an N-terminal domain with structural homology to members of the SGNH (GDSL) hydrolase superfamily and a C-terminal carbohydrate-binding module (CBM) that may bind alginate. Cells lacking this gene in the mucoid strain FRD1 are phenotypically non-mucoid, i.e. non-alginate producing, and secrete dialyzable oligouronic acids of various lengths that are mainly mannuronic acid dimers resulting from alginate lyase (AlgL) degradation of polymannuronic acid. Disruption of algX in the non-mucoid strain PAO1 does not visibly alter the phenotype of the parent. Belongs to the AlgX family.