title: Cloning and characterization of genes encoding homologues of the B subunit of cholera toxin and the Escherichia coli heat-labile enterotoxin from clinical isolates of Citrobacter freundii and E. coli.
abstract: We identified and characterized a gene encoding a homologue of the B subunits of cholera toxin (CTB) and heat-labile enterotoxin (LTB) of Escherichia coli from a clinical isolate of Citrobacter freundii that was found to produce a factor in the culture supernatant that cross-reacted with antibodies to CTB and LTB when assayed by enzyme-linked immunosorbent assay (ELISA). The gene encoding the ELISA-positive factor, cfxB, consisted of 375 nucleotides and was located downstream of an 852-nucleotide open reading frame, cfxA, with a 56-nucleotide intergenic space. The cfxB gene was predicted to encode a 125-amino-acid polypeptide, which had 73.8 and 72.8% identities with the amino acid sequences of LTB and CTB, respectively. However, the amino acid sequence of the deduced polypeptide CFXA had no homologies to those of the A subunits of CT or LT. DNA probes developed from the sequences of cfxA and cfxB were used to screen 67 C. freundii isolates and 152 E. coli isolates from diarrheal patients by colony blot hybridization. Two strains, C. freundii 48 and E. coli 176, reacted with both DNA probes under conditions of high stringency. We cloned homologues of the cfxA and cfxB genes from E. coli 176 and designated them ecxA and ecxB, respectively. The ecxA gene and the ecxB gene comprise 855 and 375 nucleotides, respectively, with a 50-nucleotide intergenic space, and encode a 285- and a 125-amino-acid residue polypeptides, respectively. The results of the present study may provide important clues to the origin and evolution of immunologically related factors sharing a common enterotoxin-like A and B subunit structures.
authors.affiliation: Department of Bacteriology, Graduate School of Medical Science, Kanazawa University, 13-1 Takara-machi, Japan. karasawa@med.kanazawa-u.ac.jp
authors.forename: Tadahiro
authors.initials: T
authors.lastname: Karasawa
authors.forename: Hideaki
authors.initials: H
authors.lastname: Ito
authors.forename: Teizo
authors.initials: T
authors.lastname: Tsukamoto
authors.forename: Shinji
authors.initials: S
authors.lastname: Yamasaki
authors.forename: Hisao
authors.initials: H
authors.lastname: Kurazono
authors.forename: Shah M
authors.initials: SM
authors.lastname: Faruque
authors.forename: G Balakrish
authors.initials: GB
authors.lastname: Nair
authors.forename: Mitsuaki
authors.initials: M
authors.lastname: Nishibuchi
authors.forename: Yoshifumi
authors.initials: Y
authors.lastname: Takeda
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D001427:Bacterial Toxins / Q000235:genetics* / Q000378:metabolism; D002675:Child, Preschool; D002772:Cholera Toxin / Q000235:genetics* / Q000378:metabolism; D016971:Citrobacter freundii / Q000235:genetics*; D003001:Cloning, Molecular; D006224:Cricetinae; D003967:Diarrhea / Q000382:microbiology; D004756:Enterobacteriaceae Infections / Q000382:microbiology; D004768:Enterotoxins / Q000235:genetics* / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D004926:Escherichia coli / Q000235:genetics*; D004927:Escherichia coli Infections / Q000382:microbiology; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007223:Infant; D051379:Mice; D017386:Sequence Homology, Amino Acid
chemical_list: D001426:Bacterial Proteins; D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D002772:Cholera Toxin; C027538:heat-labile enterotoxin, E coli