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Mouse-virulentSalmonella typhimurium strains SR-11 and ATCC 14028-1s express curli fibers, thin aggregative fibers, at ambient temperature on plates as judged by Western blot analysis and electron microscopy. Concomitantly with curli expression, cells develop a rough and dry colony morphology and bind the dye Congo red (called the rdar morphotype). Cloning and characterization of the two divergently transcribed operons required for curli biogenesis,csgBA(C) andcsgDEFG , fromS. typhimurium SR-11 revealed the same gene order and flanking genes as inEscherichia coli . The divergence of the curli region betweenS. typhimurium andE. coli at the nucleotide level is above average (22.4%). However, a high level of conservation at the protein level, which ranged from 86% amino acid homology for the fiber subunit CsgA to 99% homology for the lipoprotein CsgG, implies functional constraints on the gene products. Consequently,S. typhimurium genes on low-copy-number plasmids were able to complement respectiveE. coli mutants, although not always to wild-type levels.rpoS andompR are required for transcriptional activation of (at least) thecsgD promoter. The high degree of conservation at the protein level and the identical regulation patterns inE. coli andS. typhimurium suggest similar roles of curli fibers in the same ecological niche in the two species.

Curli Fibers Are Highly Conserved between Salmonella typhimurium and Escherichia coli with Respect to Operon Structure and Regulation