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 inactivate target proteins in their animal hosts. Recently, several vertebrate ectoenzymes (ART1-ART7) with activities similar to bacterial toxins have also been cloned. We show here that PSI-BLAST, a position-specific-iterative database search program, faithfully connects all known vertebrate ecto-mono(ADP-ribosyl)transferases (mADPRTs) with most of the known bacterial mADPRTs. intriguingly, no matches were found in the available public genome sequences of archaeabacteria, the yeast Saccharomyces cerevisiae or the nematode Caenorhabditis elegans. Significant new matches detected by PSI-BLAST from the public sequence data bases included only one open reading frame (ORF) of previously unknown function: the spvB gene contained in the virulence plasmids 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 toxin TcaC from nematode-infecting enterobacteria. We produced the predicted catalytic 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 regarding salmonella pathogenesis.