Specific insertion and deletion of insertion sequence 1-like DNA element causes the reversible expression of the virulence capsular antigen Vi of Citrobacter freundii in Escherichia coli

Citrobacter freundii strain WR7004 reversibly expresses the virulence capsular antigen Vi, whose production is controlled by two distinct chromosomal loci, viaA and viaB. The rate of oscillation between the Vi-producing (Vi⁺) state and the Vi-nonproducing (Vi^-) state in strain WR7004 ranges from 2 x 10^-4 to 7 x 10^-3 transitions per bacterium per generation. A similarly high conversion rate from Vi⁺ to Vi^- occurs in Escherichia coli HB101 harboring pWR127, a plasmid that contains the 18-kilobase-pair (kb) viaB region cloned from WR7004. However, the Vi^- state in HB101 harboring pWR127 was so stable that transition to the Vi⁺ state was not detected (<10^-10 per bacterium per generation). When pWR127 DNA derived from Vi^- strains of HB101 harboring pWR127 was transformed in HB101 recipient, a small number of Vi⁺ transformants were seen among the transformants, which were predominantly Vi^-. The viaB region consists of two EcoRI digestion fragments, A (8.6 kb) and B (9.4 kb). A discrete 700- to 800-base-pair DNA element was found to be inserted in the EcoRI B fragment of the viaB region of pWR127 when derived from Vi^- strains. However, no such DNA element was found in the pWR127 DNA isolated from Vi⁺ strains. This discrete DNA element inserts into a recombinational hot spot 1.1 kb from the end of the EcoRI B fragment and behaves as an insertion sequence 1 (IS1)-like element. The insertion of this IS1-like element in the viaB region thus disrupts expression of the Vi antigen. Restoration of Vi expression results when this element is excised.