Fim Switch on:
[Wikipedia]
[Google]
[Amazon]
The ''fim'' switch in ''
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. 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.
''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, 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, 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, in ''
Escherichia coli
''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'' is the mechanism by which the ''fim'' gene cluster, encoding Type I Pili, is transcriptionally controlled.
These pili are virulence factors involved in adhesion, especially important in uropathogenic ''Escherichia coli''. The gene undergoes phase variation In biology, phase variation is a method for dealing with rapidly varying environments without requiring random mutation. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial populati ...
mediated via two recombinases
Recombinases are genetic recombination enzymes.
Site specific recombinases
DNA recombinases are widely used in multicellular organisms to manipulate the structure of genomes, and to control gene expression. These enzymes, derived from bacteria ...
and is a model example of site specific inversion.
Structure and mechanism of phase variation
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. 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.
''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, 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, 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, in ''Salmonella typhimurium
''Salmonella enterica'' subsp. ''enterica'' is a subspecies of ''Salmonella enterica'', the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the ''S. enterica'' species are in this subspecies, includin ...
'', for example much influence is exerted by a leucine-responsive regulatory protein and there is no ''fim'' S element.
References
{{reflist Genetics Escherichia coli Gene expression Virulence factors