The Flag-2 Locus, an Ancestral Gene Cluster, Is Potentially Associated with a Novel Flagellar System from Escherichia coli

Abstract

Escherichia coli K-12 possesses two adjacent, divergent, promoterless flagellar genes, fhiA-mbhA, that are absent from Salmonella enterica. Through bioinformatics analysis, we found that these genes are remnants of an ancestral 44-gene cluster and are capable of encoding a novel flagellar system, Flag-2. In enteroaggregative E. coli strain 042, there is a frameshift in lfgC that is likely to have inactivated the system in this strain. Tiling path PCR studies showed that the Flag-2 cluster is present in 15 of 72 of the well-characterized ECOR strains. The Flag-2 system resembles the lateral flagellar systems of Aeromonas and Vibrio, particularly in its apparent dependence on RpoN. Unlike the conventional Flag-1 flagellin, the Flag-2 flagellin shows a remarkable lack of sequence polymorphism. The Flag-2 gene cluster encodes a flagellar type III secretion system (including a dedicated flagellar sigma-antisigma combination), thus raising the number of distinct type III secretion systems in Escherichia/Shigella to five. The presence of the Flag-2 cluster at identical sites in E. coli and its close relative Citrobacter rodentium, combined with its absence from S. enterica, suggests that it was acquired by horizontal gene transfer after the former two species diverged from Salmonella. The presence of Flag-2-like gene clusters in Yersinia pestis, Yersinia pseudotuberculosis, and Chromobacterium violaceum suggests that coexistence of two flagellar systems within the same species is more common than previously suspected. The fact that the Flag-2 gene cluster was not discovered in the first 10 Escherichia/Shigella genome sequences studied emphasizes the importance of maintaining an energetic program of genome sequencing for this important taxonomic group.The motile gamma-proteobacterium Escherichia coli has been widely accepted in biology as a model organism, an opinion typified by quotations such as "all cell biologists have two cells of interest: the one they are studying and Escherichia coli" (34) or Jacques Monod's famous dictum "Tout ce qui est vrai pour le Colibacille est vrai pour l'éléphant" ("What is true for E. coli is also true of the elephant") (26). However, within this single model species, which now encompasses the shigellas, there are remarkable variations in genome size, and the largest E. coli genomes possess more than 1 Mb more DNA than the smallest E. coli genomes (42). Comfortingly, the most commonly used laboratory strain, K-12, has one of the smallest E. coli genomes, leading to the often unwitting assumption that this model strain represents the ancestral or archetypical state of the species.Curiously, one area of bacteriology in which E. coli K-12 has been eclipsed as a model organism is the study of flagellar biosynthesis, assembly, and regulation. In this area, Salmonella enterica serovar Typhimurium strain LT2 has been the most commonly used model organism (34,35). Nonetheless, it has been assumed that the genetics and physiology of flagellar systems are essentially the same in E. coli and S. en...

Plasticity of the domain structure in FlgJ, a bacterial protein involved in flagellar rod formation

“…Y. pestis also possesses both dual-and single-domain FlgJ homologs, the latter of which is encoded by the flagellar gene cluster homologous to the lateral flagellar gene set of V. parahaemolyticus (Parkhill et al, 2003). After our database search presented here was finished, at least two additional bacterial strains, enteroaggregative E. coli strain 042 (Ren et al, 2005) and Aeromonas hydrophila strain AH-3 (Canals et al, 2006), have been reported to possess flagellar gene clusters homologous to the V. parahaemolyticus lateral flagellar gene set. These observations led to a hypothesis that the lateral flagellar gene set resides within a movable element and has been acquired by lateral gene transfer (Ren et al, 2005).…”

“…After our database search presented here was finished, at least two additional bacterial strains, enteroaggregative E. coli strain 042 (Ren et al, 2005) and Aeromonas hydrophila strain AH-3 (Canals et al, 2006), have been reported to possess flagellar gene clusters homologous to the V. parahaemolyticus lateral flagellar gene set. These observations led to a hypothesis that the lateral flagellar gene set resides within a movable element and has been acquired by lateral gene transfer (Ren et al, 2005). Our molecular data analyses on FlgJ described below support this hypothesis.…”

Flagellin-Dependent and -Independent Inflammatory Responses following Infection by EnteropathogenicEscherichia coliandCitrobacter rodentium

“…C. rodentium is a related A/E pathogen that infects mice, and it has recently provided a valuable animal model of human EHEC and EPEC infection and disease. While C. rodentium is described as nonmotile, its genome was recently released, and genes for two flagellins (FliC and Flag-2) were found within the C. rodentium genome (www .sanger.ac.uk/Projects/C_rodentium/) (33). To clarify whether C. rodentium is truly flagellin deficient, we assessed flagellin expression through immunoblotting using a mouse monoclonal antibody raised against pathogenic-E. coli flagella (and broadly cross-reactive against most flagellin types).…”

Analysis of the Lateral Flagellar Gene System ofAeromonas hydrophilaAH-3

“…V. parahaemolyticus is the best-studied organism, and it has two distinct flagellar systems. Recently, an Escherichia coli O42 lateral flagellar gene cluster (Flag-2) has been described (33), and the presence of Flag-2-like gene clusters in Yersinia pestis, Yersinia pseudotuberculosis, and Chromobacterium violaceum suggests that the coexistence of two flagellar systems in the same species is more common than previously suspected (33). The polar flagellum (Fla) of V. parahaemolyticus requires around 60 genes distributed in five clusters on chromosome I for biogenesis and assembly (41), whereas lateral flagella (Laf) are coded for by 38 different genes distributed in two clusters on chromosome II (39).…”