The basis of the rough-to-smooth conversion of Actinobacillus actinomycetemcomitans was examined. Smooth variants often contained mutations at the flp promoter region. Replacing the mutated flp promoter with the wild-type promoter restored the rough phenotype. The expression level of the flp promoter was approximately 100-fold lower in smooth than in rough strains. Mutations of the flp promoter are a cause of the rough-to-smooth conversion.
