Nested deletions of the SRL pathogenicity island of Shigella flexneri 2a

In this study, we determined the boundaries of a 99-kb deletable element of Shigella flexneri 2a strain YSH6000. The element, designated the multiple- antibiotic resistance deletable element (MRDE), had recently been found to contain a 66-kb pathogenicity island (PAI)-like element (designated the SRL PAI) which carries the Shigella resistance locus (SRL), encoding resistanc e determinants to streptomycin, ampicillin, chloramphenicol, and tetracycli ne. The YSH6000 MRDE was found to be flanked by two identical IS91 elements present at the S. flexneri homologs of the Escherichia coli genes putA and mdoA on NotI fragment D. Sequence data from two YSH6000-derived MRDE delet ants, YSH6000T and S2430, revealed that deletion of the MRDE occurred betwe en the two flanking IS91 elements, resulting in a single IS91 element spann ing the two original IS91 loci. Selection for the loss of tetracycline resi stance confirmed that the MRDE deletion occurred reproducibly from the same chromosomal site and also showed that the SRL PAI and the SRL itself were capable of independent deletion from the chromosome, thus revealing a uniqu e set of nested deletions. The excision frequency of the SRL PAI was estima ted to be 10(-5) per cell in the wild type, and mutation of a P4-like integ rase gene (int) at the left end of the SRL PAI revealed that int mediates p recise deletion of the PAI.