Ribose lowers RpoS translation through RbsD mRNA

The Escherichia coli sigma factor RpoS accumulates during starvation, activates transcription of the general stress response, and then rapidly returns to low levels upon nutrient replenishment. Sugar levels modulate RpoS levels by signaling though central metabolism, ATP and the phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS) transport system, which favors glucose. However, there have been few examples of dedicated control pathways for specific sugars regulating RpoS. We screened an overexpression library with a RpoS- LacZ reporter and found that overexpressing the ribose metabolism rbsD gene reduced RpoS levels. A functional RbsD ribose pyranase protein was not necessary for the effect, suggesting that the RbsD mRNA was responsible for the effect. We used a series of LacZ fusions and RT-qPCR to determine how RbsD mRNA affects RpoS and found that regulation of RpoS occurred at the level of translation. Furthermore, the effect of rbsD overexpression was diminished in strain that did not have the RpoS untranslated hairpin loop or the small RNA chaperone Hfq. RbsD mRNA has previously been shown to bind to the sRNA DsrA. We demonstrated that the effect of RbsD on RpoS was dependent on the sRNAs DsrA and ArcZ and Hfq. Finally, we showed that the sugar ribose lowers RpoS levels in a manner that requires RbsD. In summary, our results demonstrate that RpoS levels respond specifically to ribose though a component of the ribose metabolism pathway and the main player, RbsD, acts as dual function mRNA that has a regulatory role by interacting with the sRNAs that control RpoS.