Genetic Rearrangements of the Regions Adjacent to Genes Encoding Heat-Labile Enterotoxins ( eltAB ) of Enterotoxigenic Escherichia coli Strains

Abstract

One of the most common bacterially mediated diarrheal infections is caused by enterotoxigenic Escherichia coli (ETEC) strains. ETEC-derived plasmids are responsible for the distribution of the genes encoding the main toxins, namely, the heat-labile and heat-stable enterotoxins. The origins and transfer modes (intra-or interplasmid) of the toxin-encoding genes have not been characterized in detail. In this study, we investigated the DNA regions located near the heat-labile enterotoxin-encoding genes (eltAB) of several clinical isolates. It was found that the eltAB region is flanked by conserved 236-and 280-bp regions, followed by highly variable DNA sequences which consist mainly of partial insertion sequence (IS) elements. Furthermore, we demonstrated that rearrangements of the eltAB region of one particular isolate, which harbors an IS91R sequence next to eltAB, could be produced by a recA-independent but IS91 sequence-dependent mechanism. Possible mechanisms of dissemination of IS element-associated enterotoxin-encoding genes are discussed.Enterotoxigenic Escherichia coli (ETEC) infections are the major cause of bacterium-associated diarrheal diseases in developing countries (5, 15) and among travelers (4, 37). They are the subject of development of novel vaccines (19,45). The main virulence determinants of ETEC strains are heat-labile enterotoxin (LT) and heat-stable enterotoxin (ST) (2, 40) and specific colonization factors (CFs) (13). ETEC strains colonize the small intestine and encode more than 20 different CFs (13). Indeed, combinations of CFs together with either ST or ST and LT in ETEC strains are the main risk factors for acquisition of acute ETEC-associated diarrheal diseases (14, 26).About one-third of the clinically relevant ETEC strains express both LT and ST, whereas the remaining two-thirds express either LT or ST (13). LT and ST can be encoded together or separately on large, variable plasmids called Ent plasmids (42), along with CFs, antibiotic resistance markers, and conjugation systems (11,41).Genes encoding LT (8) and cholera toxin (27) presumably have a common ancestor, since considerable amino acid and DNA sequence homologies are apparent (8). It has also been proposed that the LT genes are foreign genes which were acquired by horizontal gene transfer to form an enteropathogen (33, 50). The activities and structures of LT and cholera toxin are nearly identical. Both toxins consist of two subunits, the catalytically active subunit A and the receptor domain subunit B. The toxic activity of LT is caused by the catalytic activity of subunit A, which is able to catalyze the ADP-ribosylation of protein Gs␣ in eukaryotic cells. This in turn constitutively induces adenylate cyclase to produce elevated intracellular cyclic AMP concentrations (12).LT-encoding ETEC strains can be isolated from humans (LTh or LT-I) and animals (porcine LT [LTp] or LT-II) (2, 40). Although the overall similarity is high, some distinct differences at the DNA sequence level were observed (21, 47). The authors concluded...

Allele Variants of Enterotoxigenic Escherichia coli Heat-Labile Toxin Are Globally Transmitted and Associated with Colonization Factors

“…A previous report indicated that around 130 million years ago, before V. cholerae and E. coli diverged as species, LT genes were acquired by horizontal transfer (40). Also, it has been known that the LT sequence is flanked by insertion sequence (IS) elements, similar to those found next to genes encoding fimbriae, suggesting a general mechanism for the transmission of virulence-related genes (41,42). Our data, together with the findings that ETEC strains with the same toxin-CF profile often are genetically related, suggest that LT acquisition is not due solely to horizontal gene transfer but rather is also due to lateral gene transfer.…”

Cyclic AMP Receptor Protein-Dependent Repression of Heat-Labile Enterotoxin

“…However, it is likely that CRP acts at an earlier step of promoter initiation by sterically occluding RNA polymerase's access to the Ϫ35 hexamer of eltAp. Although ETEC strains are heterogeneous, it is likely that LT-I will be negatively regulated by CRP in most, if not all, LT-I ϩ strains because the CRP binding site centered at Ϫ31.5 is a conserved feature of the toxin promoter (39).…”

IS CR Elements: Novel Gene-Capturing Systems of the 21st Century?

“…IS91 or very closely related isoforms have also been found adjacent to various other virulence genes in enteropathogenic, enterohemolytic, and enterotoxigenic strains of E. coli (16,42,109), including the eltAB toxin-encoding genes of the heat-labile enterotoxin (89). Furthermore, direct movement of eltAB genes by IS91 has been demonstrated (89). Similarly, IS1294 has been associated with virulence genes in E. coli (96), and IS801 has been discovered in close association with virulence genes in the plant pathogen Pseudomonas syringae (66,83,95).…”

“…For example, in Argentina, the bla CTX-M-2 enzyme alone now accounts for almost 69% of all ESBLs in Enterobacteriaceae that are analyzed in Buenos Aires (30). In both instances, the resistance genes are closely associated with ISCR1 (13,89).…”

“…The Hly plasmids belonged to different incompatibility groups, which suggested that IS91 was involved in the dissemination of these pathogenicity determinants (21,37,53,114). IS91 or very closely related isoforms have also been found adjacent to various other virulence genes in enteropathogenic, enterohemolytic, and enterotoxigenic strains of E. coli (16,42,109), including the eltAB toxin-encoding genes of the heat-labile enterotoxin (89). Furthermore, direct movement of eltAB genes by IS91 has been demonstrated (89).…”