CHARACTERIZATION OF THE GENES-CODING FOR THE PUTATIVE SIGMA-FACTOR ALGU AND ITS REGULATORS MUCA, MUCB, MUCC, AND MUCD IN AZOTOBACTER-VINELANDII AND EVALUATION OF THEIR ROLES IN ALGINATE BIOSYNTHESIS

The study of the biosynthesis of alginate, the exopolysaccharide produ ced by Azotobacter vinelandii and Pseudomonas aeruginosa, has biotechn ological and medical significance. We report here the identification o f the A. vinelandii genes coding for the putative sigma factor AlgU an d its negative regulators MucA and MucB through the suppression of the highly mucoid phenotype of an A. vinelandii strain by a plasmid encod ing MucA and MucB. The sequences of the A. vinelandii algU, mucA, and mucB genes are highly homologous to those of the corresponding P. aeru ginosa genes, AlgU shows 93% identity, and MucA and MucB are 64.4 and 63.9% identical, respectively, Forming part of the same operon as algU , mucA, and mucB, two additional genes (mucC and mucD) were identified and sequenced; the product of the former gene is homologous to ORF4 o f Photobacterium sp. strain SS9, and that of the latter gene belongs t o the HtrA serine protease family. Interestingly, the nonmucoid A. vin elandii UW136 had a 0.9-kb insertion within the algU gene. A strong co rrelation between AlgU activity and alginate production by A. vineland ii was also found, as reflected in the level of algD transcription.