Summary

The conversion to mucoid, exopolysaccharide alginate-overproducing phenotype in Pseudomonas aeruginosa during chronic respiratory infections in cystic fibrosis patients occurs via mutations that activate the alternative sigma factor AlgU (sE). In this study, we demonstrate that conversion to mucoidy can be caused via a second, algU-independent path-way, in which alginate production and transcription of the critical algD promoter depend on another alternative s factor, RpoN (s54). The algD promoters dependent on s54 and sE showed a complete overlap resulting in identical mRNA 5 0 ends. The two path-ways were not independent, as s54 also repressed sE-dependent transcription of algD both in vitro and in vivo. The negative regulatory effect of s54 on sE-dependent algD expression was based on s54 binding to the algD promoter and its interference with sE-dependent transcription. This phenomenon, referred to here as s factor antagonism, reflects the unique properties of s54, which lacks an intrinsic ability to form open transcription initiation complexes. We propose that this peculiar feature of s54 has evolved in part to allow its recruitment as a repressor of certain promoter subsets. The repression of algD by s54 also depends on environmental conditions, supporting the notion that s factor antagonism plays a physiological role in controlling alginate production in P. aeruginosa during adaptation to different ecological sites (e.g. biofilm development, stress and other growth conditions) and unique environments in the chronically infected host