Characterization of the mutS-proximal region of the Salmonella typhimuriumSPI-1 identifies a group of pathogenicity island-associated genes

The virulence properties of Salmonella enterica are largely encoded within a set of horizontally acquired gene blocks termed pathogenicity islands. On e such pathogenicity island, SPI-1, located at centisome 63 of the Salmonel la chromosome between the mutS and fhIA genes, encodes a type III protein s ecretion system and an iron uptake system. We have characterized the mutS-p roximal border of this pathogenicity island and have identified two sets of genes, pigAB and pigCD. All four genes have homologs of unknown function i n several bacteria that share the ability to establish an intimate associat ion with higher eukaryotic hosts. The expression of at least two of these g enes, pigA and pigB, is controlled by SprA, a transcription factor encoded within SPI-1 that controls the expression of genes associated with the type III secretion system of this island. In addition, we found that homologs o f the pig genes are also found at different locations of the S. enterica ch romosome in association with segments of DNA that exhibit features of patho genicity islands. The presence of several apparently functional copies of t hese genes argues for an important role in the biology of this bacterial pa thogen. Furthermore. they constitute a valuable tool to identify potential pathogenicity islands. (C) 2001 Federation of European Microbiological Soci eties. Published by Elsevier Science B.V. Ail rights reserved.