SEQUENCE OF A VARIANT SHIGA-LIKE TOXIN TYPE-I OPERON OF ESCHERICHIA-COLI O111H-

PCR amplification was used to screen faecal isolates of Escherichia co li from a 12-month-old boy with haemolytic uraemic syndrome for the pr esence of Shiga-like toxin (SLT)-encoding genes. One isolate, belongin g to serotype 0111:H- was positive for SLT-I by this method. UV induct ion indicated that the strain was lysogenic for a lambdoid bacteriopha ge, but this did not encode the toxin. Southern hybridization analysis of chromosomal DNA revealed that the SLT-I gene was located on an 8.5 -kb EcoRI fragment. SLT-I was further localized to within a 3.0-kb Sph I-EcoRI fragment. A separate subclone contained a 3.75-kb HindIII frag ment, 1.18 kb of which was common to both. Nucleotide sequence analysi s of derivatives of these clones revealed that the SLT-I A subunit gen e from E. coli 0111:H- differed from the previously published sequence s for SLT-I by 5 bp resulting in two amino acid (aa) changes!. It was more closely related to the gene encoding the A subunit of the Shiga toxin from Shigella dysenteriae type 1, from which it differed by 3 bp (resulting in one aa change). The DNA sequence of the B subunit-encod ing gene was identical to that of the other two toxins. The region of DNA upstream from the SLT-I of E. coli 0111:H- contained an IS element , as well as a region with strong homology to a portion of the genome of bacteriophage lambda.