The spvB gene-product of the Salmonella enterica virulence plasmid is a mono(ABP-ribosyl)transferase

A number of well-known bacterial toxins ADP-ribosylate and thereby inactiva te target proteins in their animal hosts. Recently, several vertebrate ecto enzymes (ART1-ART7) with activities similar to bacterial toxins have also b een cloned. We show here that PSI-BLAST, a position-specific-iterative data base search program, faithfully connects all known vertebrate ecto-mono(ADP -ribosyl)transferases (mADPRTs) with most of the known bacterial mADPRTs. i ntriguingly, no matches were found in the available public genome sequences of archaeabacteria, the yeast Saccharomyces cerevisiae or the nematode Cae norhabditis elegans. Significant new matches detected by PSI-BLAST from the public sequence data bases included only one open reading frame (ORF) of p reviously unknown function: the spvB gene contained in the virulence plasmi ds of Salmonella enterica. Structure predictions of SpvB indicated that it is composed of a C-terminal ADP-ribosyltransferase domain fused via a poly proline stretch to a N-domain resembling the N-domain of the secretory toxi n TcaC from nematode-infecting enterobacteria. We produced the predicted ca talytic domain of SpvB as a recombinant fusion protein and demonstrate that it, indeed, acts as an ADP-ribosyltransferase. Our findings underscore the power of the PSI-BLAST program for the discovery of new family members in genome databases. Moreover, they open a new avenue of investigation regardi ng salmonella pathogenesis.