The role of integration host factor (IHF) in the regulation of alginate synthesis was investigated in a mucoid strain of Pseudomonas aeruginosa (strain CHA) isolated from a cystic fibrosis patient. Escherichia coli strain BL21(DE3) was made IHF-deficient by inactivation of its chromosomal IHF genes, himA and himD, then used as host strain to overproduce P. aeruginosa IHF. The purified recombinant IHF protein was used to determine the affinity of IHF for the two IHF binding sites in the algD promoter. The Kd values were determined to be 130 nM for algD IHF site 2 and about 2 microM for algD IHF site 1. Two IHF-deficient mutants of P. aeruginosa strain CHA were constructed by insertional inactivation of the himA gene, and the activity of the algD promoter was determined using transcriptional fusion with xylE as reporter gene. The expression of algD, the structural gene for GDP-mannose dehydrogenase, was decreased three- to fourfold in the himA mutants under conditions of high salinity and nitrogen limitation. Assays of alginate production by cultures grown on agar plates indicated that the IHF-deficient mutants synthesized 50% less polymer than the mucoid parental strain. These results demonstrate clearly that although IHF is dispensable for alginate production, himA expression is required for full activation of algD expression.