Inversion-independent phase variation of type 1 fimbriae in Escherichia coli

Abstract

The roles of fimB and fimE in the phase-variable expression of type 1 fimbriae in Escherichia coli were examined. A method was developed to study the effects offimB and fimE on both recombination of the fim invertible element and fimbrial expression. The method used an allelic exchange procedure consisting of two steps. The first step, construction of intermediate strains, deletedfimB andfimE. This step locked the invertible element in either the on or the off orientation. The second step of the exchange procedure introduced either wild-type or mutant alleles offimB and/orfimE into the chromosome of the intermediate strains. Analysis of the resulting strains supported the current, plasmid-based model of recombination. Unexpectedly, strains in which the invertible element was locked in the on orientation (either by mutation of bothfimB andfimE or, in a control strain, by mutation of the left inverted repeat sequence of the invertible element) continued to exhibit phase-variable expression of type 1 fimbriae. A strain in which fimA was transcribed from the tac promoter continued to exhibit phase-variable fimbrial expression, suggesting that inversion-independent phase variation cannot be explained by variable transcription initiation offimA.The expression of type 1 fimbriae in Escherichia coli exhibits on-off phase variation. This phase variation correlates with the orientation of a short, invertible DNA sequence located immediately upstream of finiA, the gene encoding the major fimbrial subunit (1, 15, 18). The promoter for fimA is believed to reside within this invertible element and to direct the transcription offimA when the element is in one orientation ("on") but not when the element is in the alternate orientation ("off'). Furthermore, the orientation of thefim invertible element correlates with the Fim phenotype (8, 18).Two genes, fimB and fimE, map immediately adjacent to the fim invertible element and are believed to encode sitespecific recombinases (8,24,29,35,36,38). The current model of the roles of fimB andfimE in inversion suggests that fimB promotes recombination of thefim invertible element in both directions, whereas fimE promotes recombination primarily from on to off (29). However, this model is based on a recombinant plasmid-based assay with which fimbrial expression cannot be examined (29).In the present study, the roles of fimB and fimE in the phase-variable expression of type 1 fimbriae were examined by use of a method that preserved the native location, organization, stoichiometry, and topology of the recombination elements. Thus, in contrast to the plasmid-based assay (29), in this assay both recombination and fimbrial expression could be examined. Analysis of the recombination results supported the current, plasmid-based model of the roles offimB andfimE. Additionally, examination of fimbrial production indicated that the on orientation of the fim invertible element was necessary but not sufficient for fimbrial expression; strains in which the invertible element was * Corresponding autho...

The leucine-responsive regulatory protein binds to the fim switch to control phase variation of type 1 fimbrial expression in Escherichia coli K-12

“…Although inversion of thefim switch is considered to be controlled by the products of fimB, fimE, himA, himD, and hns (7,8,13,(14)(15)(16), little is known about the mechanism of the site-specific inversion and its potential regulation. We reported previously that rather than being slow and random, the fim switch is capable of high inversion frequencies (>0.7 per cell per generation) and is regulated by environmental conditions, including temperature and the amino acids alanine, leucine, isoleucine, and valine (11).…”

Integrated regulatory responses of fimB to N -acetylneuraminic (sialic) acid and GlcNAc in Escherichia coli K-12

“…Likewise, the rate of FimB catalyzed off-to-on inversion was reduced Ͼ35-fold in its presence [40.8 ϫ 10 Ϫ4 per cell per generation (n ϭ 8; range of 160-3.4 ϫ 10 Ϫ4 ) to 1.1 ϫ 10 Ϫ4 per cell per generation (n ϭ 8; range 2.1-0.4 ϫ 10 Ϫ4 )]. Because only FimB catalyses off-to-on recombination at a detectable rate (19), GlcNAc inhibits the phase variation of type 1 fimbriation from the afimbriate-tofimbriate phase.…”

“…Phase variation in bacteria is determined by various mechanisms including insertion and deletion of short sequence elements by mismatch repair, deoxyadenosine methylase (Dam) methylation-dependent alternative nucleoprotein complexes, and DNA rearrangements (17). Phase variation of fim is associated with inversion of a short (Ϸ300 bp) DNA element that contains a promoter for the structural genes (18,19). Inversion of the fim element involves two tyrosine family recombinase proteins, FimB and FimE, and is subject to elaborate control producing both (i) a relatively low frequency of switching from the afimbriate (off) to fimbriate (on) phase (between 10 Ϫ4 and 10 Ϫ3 per cell per generation), and (ii) control by environmental signals, including temperature, the branched-chain amino acids (particularly leucine) and alanine, and Neu 5 Ac (20)(21)(22).…”

Differential codon usage: a safeguard against inappropriate expression of specialized genes?

“…ThetimA gene, encoding the principal type 1 fimbrilin, lacks UUG codons altogether [25]. However, synthesis of type 1 fimbriae is subject to phase variation due to inversion of a 314 bp DNA segment that includes thefimA promoter ( [26]; but see also [27]). The ratio of the products of two tim genes, timB and tmE, determine the frequencies of inversion in the two opposing directions with high levels of FimB favoring the 'off' to 'on' transition.…”