Integration host factor and sequences downstream of the Pseudomonas aeruginosa algD transcription start site are required for expression

Abstract

Pseudomonas aeruginosa is an extremely important opportunistic pathogen in immunocompromised individuals. Strains of P. aeruginosa isolated from chronic lung infections in patients with the genetic disease cystic fibrosis have a mucoid colony morphology. This phenotype is due to overproduction of the exopolysaccharide alginate, which is believed to confer a selective advantage on P. aeruginosa in cystic fibrosis lungs. Alginate biosynthesis is controlled by a complex regulatory mechanism. Genes located in the 34-min region of the P. aeruginosa chromosome form an operon which encodes most of the biosynthetic enzymes necessary for alginate production. algD, the first gene in the operon and a critical point for the transcriptional regulation of alginate biosynthesis, is controlled by several trans, cis, and environmental factors. In this study, the involvement of the histone-like protein integration host factor (IHF) in algD expression was examined. Sequences with similarity to consensus IHF-binding sites of Escherichia coli were identified 75 bp upstream (site 1) and 90 bp downstream (site 2) of the start of algD transcription. In gel band mobility shift assays, DNA fragments containing either site bind IHF but site 2 has an approximately 90-fold higher affinity for IHF. Mutations in each of the elements were generated, and they resulted in the reduction or loss of in vitro IHF binding and a three- to fourfold decrease in algD-cat expression. This indicates that IHF binding is necessary for high-level algD transcription. The presence of a high-affinity IHF-binding site located 3' of the algD transcription start site suggested that sequences further downstream of this element are involved in algD expression. When a fragment located downstream of site 2 and upstream of the promoterless cat gene (+110 to +835) was deleted, algD-cat expression was reduced 10-fold supporting the notion that 3' enhancer elements are required for algD transcription. This is the first direct evidence of a 3' element involved in the control of a P. aeruginosa gene. It is postulated that IHF mediates the formation of a higher-order looped structure which is necessary for efficient algD transcription.