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The ability to bind Congo red (Crb+) is associated with virulence of Shigella flexneri and is encoded by a large, 220-kilobase plasmid. We cloned fragments of this plasmid to isolate the sequences encoding Congo red binding, to determine the degree of conservation of these sequences among S. flexneri strains, and to study the molecular basis for loss of the Crb+ phenotype. At least two separate BamHI fragments cloned into plasmid vectors encode Congo red binding in E. coli or S. flexneri. One Crb+ clone, pTKS2, contains a copy of IS1 adjacent to the crb sequences. IS1 appears to be responsible for deletions leading to loss of Congo red binding in this clone. In addition, this clone was found to integrate into the chromosome at relatively high frequency. Integration resulted in loss of the Crb+ phenotype. A second clone, pTKS15, which has only limited homology to pTKS2, also encodes Congo red binding. The Crb+ phenotype of transformants carrying pTKS15 was detected at 37 degrees C but not at 30 degrees C, and thus it resembles Congo red binding in wild-type S. flexneri. HindIII digests of plasmid DNA from 10 different S. flexneri strains were hybridized to both of these Crb+ clones and to an IS1 probe. More than one fragment hybridized to pTKS2 or pTKS15. In general, the sizes of these fragments were the same in S. flexneri strains of different serotypes, indicating conservation of these sequences. Three of five copies of IS1 were also found on the large S. flexneri plasmids. Two of the copies were on fragments of the same size in each strain. Analysis of Crb- derivatives of the 10 strains indicated that, although IS1 may be closely linked to crb sequences on the 220-kilobase plasmid, it is not responsible for the majority of deletions of this plasmid associated with loss of Congo red binding.

infection and immunity

Characterization of Shigella flexneri sequences encoding congo red binding (crb): conservation of multiple crb sequences and role of IS1 in loss of the Crb+ phenotype