The role of integration host factor (IHF) in the regulation of alginate synthesis was investigated in a mucoid strain of (strain CHA) isolated from a cystic fibrosis patient. strain BL21 (DE3) was made IHF-deficient by inactivation of its chromosomal IHF genes, and then used as host strain to overproduce IHF. The purified recombinant IHF protein was used to determine the affinity of IHF for the two IHF binding sites in the promoter. The values were determined to be 130 nM for IHF site 2 and about 2 µM for IHF site 1. Two IHF-deficient mutants of strain CHA were constructed by insertional inactivation of the gene, and the activity of the promoter was determined using transcriptional fusion with as reporter gene. The expression of the structural gene for GDP-mannose dehydrogenase, was decreased three- to fourfold in the 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, expression is required for full activation of expression.


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