The Rcs regulon in Erwinia amylovora

We conducted genome-wide microarray analyses to determine the regulons of RcsB and RcsC in liquid medium and, for the first time, on immature pear fruit. Our array analyses identified a total of 648 genes differentially regulated by the RcsCB in vitro and in vivo. Consistent with our previous findings, RcsB acts as a positive regulator in both conditions, while RcsC positively controls amylovoran biosynthetic gene expression in vivo, but negatively in vitro. Besides amylovoran biosynthesis and regulatory genes, cell wall and cell envelope (membrane) as well as regulatory genes were the major components of the RcsBC regulon, including many novel genes. In addition, we have also demonstrated that transcripts of rcsA, rcsC and rcsD genes, but not rcsB gene, were up-regulated when grown in minimal medium or after infection of pear fruits compared to LB medium. Furthermore, a hidden Markov model (HMM) has predicted 60 genes with candidate RcsB binding site in the intergenic regions of the E. amylovora ATCC 49946 genome and 18 (28) of them were identified in the microarray assay. Based on our findings, a working model has been proposed to illustrate how the Rcs phosphorelay system regulates virulence gene expression in E. amylovora. A total of 12 samples were analyzed in two conditions: For in vitro condition (MBMA medium + 1% sorbitol), Erwinia amylovora wild type strain (2 replicates); E. amylovora rcsB mutant strain (2 replicates); E. amylovora rcsC mutant strain (2 replicates): For in vivo condition (on wounded immature pear fruits), Erwinia amylovora wild type strain (2 replicates); E. amylovora rcsB mutant strain (2 replicates); E. amylovora rcsC mutant strain (2 replicates).