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The  p hosphate- s pecific  t ransport operon,  pstSCAB - phoU , of Gram-negative bacteria is an essential part of the Pho regulon. Its key roles are to encode a high-affinity inorganic phosphate transport system and to prevent activation of PhoB in phosphate-rich environments. In general, mutations in  pstSCAB-phoU  lead to the constitutive expression of the Pho regulon. Previously, we constructed a  pstCA  deletion mutant of  Citrobacter rodentium  and found it to be attenuated for virulence in mice, its natural host. This attenuation was dependent on PhoB or PhoB-regulated gene(s) because a  phoB  mutation restored virulence for mice to the  pstCA  mutant. To investigate how downstream genes may contribute to the virulence of  C. rodentium , we used microarray analysis to investigate global gene expression of  C. rodentium  strain ICC169 and its isogenic  pstCA  mutant when grown in phosphate-rich medium. Overall 323 genes of the  pstCA  mutant were differentially expressed by at least 1.5-fold compared to the wild-type  C. rodentium . Of these 145 were up-regulated and 178 were down-regulated. Differentially expressed genes included some involved in phosphate homoeostasis, cellular metabolism and protein metabolism. A large number of genes involved in stress responses and of unknown function were also differentially expressed, as were some virulence-associated genes. Up-regulated virulence-associated genes in the  pstCA  mutant included that for DegP, a serine protease, which appeared to be directly regulated by PhoB. Down-regulated genes included those for the production of the urease, flagella, NleG8 (a type III-secreted protein) and the  tad  focus (which encodes type IVb pili in  Yersinia enterocolitica ). Infection studies using C57/BL6 mice showed that DegP and NleG8 play a role in bacterial virulence. Overall, our study provides evidence that Pho is a global regulator of gene expression in  C. rodentium  and indicates the presence of at least two previously unrecognized virulence determinants of  C. rodentium , namely, DegP and NleG8.

Genome-Wide Analysis of the Pho Regulon in a pstCA Mutant of Citrobacter rodentium