Previous data indicated thatCystic fibrosis patients, burn wound victims, and the immunocompromised are particularly susceptible to infection by Pseudomonas aeruginosa, a Gram-negative opportunistic pathogen (1). A number of virulence determinants contribute to the pathogenesis of P. aeruginosa, including two ADP-ribosyltransferases, Exotoxin A and Exoenzyme S (ExoS) 1 (2). ExoS is a 49-kDa protein (3) that is secreted by the type III mechanism of P. aeruginosa (4), and ADP-ribosylates a number of target proteins in vitro including apolipoprotein A1, IgG3 (5), vimentin (6), and several members of the Ras superfamily (7). In vivo, ExoS is a cytotoxin (8) that ADP-ribosylates Ras during the course of infection in cultured cells (9), disrupting Ras-mediated signal transduction (10). Indirect methods were used to determine that Exoenzyme S ADP-ribosylates c-Ha-Ras at multiple sites (10).Several vertebrate ADP-ribosyltransferases have been identified to date. The vertebrate ADP-ribosyltransferases possess specific properties that are distinct from the bacterial ADPribosyltransferases. Several of the vertebrate ADP-ribosyltransferases have the capacity to ADP-ribosylate multiple target proteins. For example, a murine lymphocyte transferase ADP-ribosylates a number of cell surface proteins (11), whereas rabbit skeletal muscle ADP-ribosyltransferase ADP-ribosylates several proteins in skeletal muscle T-tubules (12). In addition, several vertebrate ADP-ribosyltransferases modify target proteins at multiple sites. Both turkey erythrocyte type A ADP-ribosyltransferase (13) and chicken ADP-ribosyltransferase (14) ADP-ribosylate actin at two arginine residues. These sites of ADP-ribosylation are located in different regions of actin, which suggests that each ADP-ribosylation event is independent. Although the role of endogenous ADP-ribosylation in the regulation of eukaryotic cell physiology has not been defined in detail, the RT6 transferase appears to be involved in the regulation of T-lymphocyte function (15), and endogenous ADP-ribosylation may contribute to hippocampal long-term potentiation (16).In this study, the ADP-ribosylation of Ras by Exoenzyme S was characterized. Using both mass spectrometry and rpHPLC coupled with Edman degradation, it was determined that ExoS ADP-ribosylates Ras at arginines 41 and 128. These residues are located in a -sheet and ␣-helix, respectively, which are spatially separated in Ras. Thus, ExoS has functional similarities to the eukaryotic ADP-ribosyltransferases because ExoS ADP-ribosylates a number of target proteins in vitro and ADPribosylate Ras at two nonadjacent arginine residues. In addition, homology studies have identified more primary sequence homology between ExoS and the vertebrate ADP-ribosyltransferases than the bacterial ADP-ribosyltransferases.