Fig. 4. The RTX toxin does not modify the Rho GTPase similarly to C.difficile toxin A (CdA). (A) A PSI-BLAST search (Altschul et al., 1997) comparing RtxA with the DDBJ/EMBL/GenBank database detected a region of limited sequence similarity to CdA as shown (score = 46.9, E = 0.002). (B) To check for a CdA-like modification of Rho in Bah1P-infected cells, Rho protein in 50 µg of cell extract was ADP-ribosylated with exoenzyme C3 in vitro. Prior modification of Rho by CdA protects cells from further modification by C3 as shown in lane 8.