Isolation, characterization, and mapping of Tn5 insertions into the 140-megadalton invasion plasmid defective in the mouse Sereny test in Shigella flexneri 2a

Using Shigella flexneri 2a YSH6000, we isolated 304 independent Tn5 insertion mutants in the 230-kilobase invasion plasmid, pMYSH6000. The site of each Tn5 insertion was assigned to 23 SalI fragments on the previously made SalI cleavage map of pMYSH6000. Among the 304 insertions, 150 were negative in expression of four phenotypes examined (mouse Sereny test [Ser], invasion into epithelial cells [Inv], Congo red binding [Pcr], and inhibition of bacterial growth [Igr] ): 12 were Ser- Inv+ Pcr+ Igr+, and 142 were positive in all four phenotypes. Tn5 insertions in the avirulent mutants were distributed in two separate SalI fragments, F and G, and in four contiguous SalI fragments, B, P, H, and D. Fragment G contains a novel class of determinant(s) which is required only for Ser+ but not for Inv+, Pcr+, and Igr+. Fragment F contains the previously characterized virF locus. B, P, H, and D each contained both virulent and avirulent Tn5 insertions. This indicates that more than two gene clusters exist within this region. Both are required for expression of all four virulence phenotypes.