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J Bacteriol, February 1998, p. 722-731, Vol. 180, No. 3
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

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

Ute Römling,1,* Zhao Bian,1 Mårten Hammar,1 Walter D. Sierralta,2 and Staffan Normark1

Department of Bacteriology, Microbiology and Tumorbiology Center, Karolinska Institutet, S-17177 Stockholm, Sweden,1 and Max-Planck-Institut für Experimentelle Endokrinologie, D-30603 Hannover, Germany2

Received 27 August 1997/Accepted 12 November 1997

Mouse-virulent Salmonella 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) and csgDEFG, from S. typhimurium SR-11 revealed the same gene order and flanking genes as in Escherichia coli. The divergence of the curli region between S. typhimurium and E. 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 respective E. coli mutants, although not always to wild-type levels. rpoS and ompR are required for transcriptional activation of (at least) the csgD promoter. The high degree of conservation at the protein level and the identical regulation patterns in E. coli and S. typhimurium suggest similar roles of curli fibers in the same ecological niche in the two species.

* Corresponding author. Mailing address: Karolinska Institutet, Microbiology and Tumorbiology Center (MTC), Box 280, S-17177 Stockholm, Sweden. Phone: (46) 8-728-7409. Fax: (46) 8-342651. E-mail: ute.roemling{at}mtc.ki.se.




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