Construction and characterization of Pseudomonas aeruginosa algB mutants: role of algB in high-level production of alginate

The algB gene, which is involved in the production of alginate in Pseudomonas aeruginosa, was localized to approximately 2.2 kilobases of DNA from strain FRD by using transposon Tn501 insertion mutagenesis, subcloning, and complementation techniques. The previously reported alg-50(Ts) mutation, which confers the phenotype of temperature-sensitive alginate production, was here designated as an algB allele. A transduction-mediated gene replacement technique was used for site-directed mutagenesis to isolate and characterize algB::Tn501 mutants of P. aeruginosa FRD. Although algB::Tn501 mutants had a nonmucoid phenotype (indicating an alginate deficiency), they still produced about 1 to 5% of wild-type levels of alginate in most growth media and up to 16% in very rich media. The algB::Tn501 mutations had no apparent effect on growth rate or growth requirements. Using another gene replacement technique called excision marker rescue, we constructed a chromosomal algB deletion (delta algB) mutant of P. aeruginosa FRD. The delta algB mutant also produced low levels of alginate as did the algB::Tn501 mutants. The alginate produced by algB::Tn501 mutants resembled wild-type alginate by all criteria studied: molecular weight, acetylation, and proportion of mannuronic and guluronic acids. Thus, the algB gene product is apparently involved in the high-level production of alginate by P. aeruginosa and is not directly involved in the pathway leading to its biosynthesis. Chromosomal mapping of an algB::Tn501 insertion showed linkage to the trp-2 marker on the FRD chromosome as does the algB50(Ts) mutation. The excision marker rescue technique was also used to place the algB::Tn501 marker on the chromosome of characterized strains of P. aeruginosa PAO. The algB::Tn501 mutation mapped near 21 min on the PAO chromosome.