Phase Variation of the mrp Fimbrial Promoter

Mannose-resistant Proteus-like (MR/P) fimbriae, produced by the uropathogen Proteus mirabilis, are major contributors to urinary tract infection. Expression of mrp genes is controlled by an invertible element in the mrp operon promoter, and promoter orientation is controlled by a single recombinase, MrpI, which determines whether the element is in the on or off orientation. Detailed here is a simple assay to determine the orientation of the invertible element in a population of P. mirabilis, a rapid screen to detect element-locked mrpI mutants, and quantification of mixtures of on and off bacteria.