Construction of strain WRSS1. (A) Plasmid pΔvirG contains a virG gene with a 212-bp deletion cloned into pCVD442. The deleted version is shaded to distinguish it from the wild-type gene. Primer BA76 is located in the oriR6K portion of pCVD442. (B) The virG gene located on the invasion plasmid of S. sonnei Mosely with the positions of the primers used to monitor recombinants. BA118 is located in the 5′ noncoding region of virG. (C) The product of the first recombination event. Primers BA118 and BA76 were used to monitor insertion of the pΔvirG plasmid into virG. (D) The product of the second recombination event generating WRSS1. Sequences homologous to primers BA114 and BA117 are present in panels A, B, C, and D. Sequences homologous to primer BA118 are present only in panels B, C, and D. Sequences homologous to primers BA76 are present only in panels A and C.