IDENTIFICATION OF A CHROMOSOMAL SHIGELLA-FLEXNERI MULTI-ANTIBIOTIC RESISTANCE LOCUS WHICH SHARES SEQUENCE AND ORGANIZATIONAL SIMILARITY WITH THE RESISTANCE REGION OF THE PLASMID NR1

The ampicillin resistance gene from Shigella flexneri 2a strain YSH600 0 was cloned and shown by Southern hybridization analysis to be closel y linked to the previously cloned streptomycin, chloramphenicol, and t etracycline resistance determinants, which are borne on a chromosomall y integrated 99-kb element. Analysis of this chromosomal multi-antibio tic resistance locus revealed that it had a high level of sequence and organizational similarity to an equivalent region of the Shigella R-p lasmid, NR1. However, the chromosomal locus exhibited several differen ces, including the presence of two stretches of sequence derived from IS elements, the precise insertion of a beta-lactamase encoding oral c assette into the Tn21-borne integron In2, a possible 17.5-kb deletion, and the loss or inactivation of the mercury resistance determinant. B ased on these data, it is proposed that the chromosomal locus arose fo llowing integration of an NR1-like plasmid. (C) 1997 Academic Press.