Role of the stress sigma factor RpoS in GacA/RsmA-controlled secondary metabolism and resistance to oxidative stress in Pseudomonas fluorescens CHA0

In Pseudomonas fluorescens biocontrol strain CHA0, the two-component system GacS/GacA positively controls the synthesis of extracellular products such as hydrogen cyanide, protease, and 2,4-diacetylphloroglucinol, by upregulating the transcription of small regulatory RNAs which relieve RsmA-mediated translational repression of target genes. The expression of the stress sigma factor σS (RpoS) was controlled positively by GacA and negatively by RsmA. By comparison with the wild-type CHA0, both a gacS and an rpoS null mutant were more sensitive to H2O2 in stationary phase. Overexpression of rpoS or of rsmZ, encoding a small RNA antagonistic to RsmA, restored peroxide resistance to a gacS mutant. By contrast, the rpoS mutant showed a slight increase in the expression of the hcnA (HCN synthase subunit) gene and of the aprA (major exoprotease) gene, whereas overexpression of σS strongly reduced the expression of these genes. These results suggest that in strain CHA0, regulation of exoproduct synthesis does not involve σS as an intermediate in the Gac/Rsm signal transduction pathway whereas σS participates in Gac/Rsm-mediated resistance to oxidative stress.