Fim switch
The fim switch in Escherichia coli is the mechanism by which the fim gene cluster, encoding Type I Pili,[1] is transcriptionally controlled.
These pili are virulence factors involved in adhesion, especially important in uropathogenic Escherichia coli. The gene undergoes phase variation mediated via two recombinases and is a model example of site specific inversion.
Structure and mechanism of phase variation
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The operon consists of the promoter region fim S, the main constituent fim A, its gene product forming a rod like structure and fim H, coding for an adhesin at the tip, to name just a few important elements. The fim S region is flanked by 9bp repeats that are mirror images of each other.[2] These mirror images serve as substrates for two ATP-dependent recombinases, fim B and fim E. These recombinases can invert the orientation of the fim S region and only one orientation allows for 3' to 5' transcription.[citation needed]
fim B "flips" the promoter region both ways, from the "on" position to the "off" position and vice versa, whereas fim E can only facilitate recombination from "on" to "off". This equilibrium, shifted towards maintaining the "off" position, due to higher fim E activity,[3] serves as a mode of expressing pili only when adhesion is needed. Another level of transcriptional control in E. coli is mediated by the sensitivity of the recombinases to pH and osmolarity,[4] further ensuring appropriate expression levels of type-I pili, given the stark differences in osmolarity inside and outside an animal's body. Type-I pili are expressed by many species of Enterobacteriaceae. The transcriptional control can differ widely between species,[5] in Salmonella typhimurium, for example much influence is exerted by a leucine-responsive regulatory protein and there is no fim S element.[5]
References
[edit]- ^ Klemm, P (1986). "Two regulatory fim genes, fimB and fimE, control the phase variation of type 1 fimbriae in Escherichia coli". The EMBO Journal. 5 (6): 1389–1393. doi:10.1002/j.1460-2075.1986.tb04372.x. ISSN 0261-4189. PMC 1166953. PMID 2874022.
- ^ Abraham, J. M.; Freitag, C. S.; Clements, J. R.; Eisenstein, B. I. (1985). "An invertible element of DNA controls phase variation of type 1 fimbriae of Escherichia coli". Proceedings of the National Academy of Sciences. 82 (17): 5724–5727. Bibcode:1985PNAS...82.5724A. doi:10.1073/pnas.82.17.5724. ISSN 0027-8424. PMC 390624. PMID 2863818.
- ^ Holden, Nicola; Blomfield, Ian C.; Uhlin, Bernt-Eric; Totsika, Makrina; Kulasekara, Don Hemantha; Gally, David L. (Dec 2007). "Comparative analysis of FimB and FimE recombinase activity". Microbiology. 153 (Pt 12): 4138–4149. doi:10.1099/mic.0.2007/010363-0. ISSN 1350-0872. PMID 18048927.
- ^ Schwan, William R.; Lee, Jeffrey L.; Lenard, Farrah A.; Matthews, Brian T.; Beck, Michael T. (2002). "Osmolarity and pH Growth Conditions Regulate fim Gene Transcription and Type 1 Pilus Expression in Uropathogenic Escherichia coli". Infection and Immunity. 70 (3): 1391–1402. doi:10.1128/IAI.70.3.1391-1402.2002. ISSN 0019-9567. PMC 127777. PMID 11854225.
- ^ a b McFarland, Kirsty A.; Lucchini, Sacha; Hinton, Jay C. D.; Dorman, Charles J. (2008). "The Leucine-Responsive Regulatory Protein, Lrp, Activates Transcription of the fim Operon in Salmonella enterica Serovar Typhimurium via the fimZ Regulatory Gene". Journal of Bacteriology. 190 (2): 602–612. doi:10.1128/JB.01388-07. ISSN 0021-9193. PMC 2223685. PMID 17981960.