In Azotobacter vinelandii , the sigma factor RpoS is maintained at low levels in exponentially growing cells due to degradation mediated by the chaperone–protease complex ClpXP, while high levels are observed in the stationary phase. This study showed that degradation of RpoS by ClpXP is under the control of the Gac-Rsm signal transduction system, in which GacA, the transcriptional activator of the two-component system GacS/GacA, activates transcription of the small RNAs RsmZ1 and RsmZ2. These RNAs bind to the translational repressor protein RsmA to counteract its repressor activity on its target mRNAs. We found that in stationary-phase cells, compared with the WT, a gacA mutant exhibited lower RpoS levels due to reduced stability, while levels of the clpP and clpX mRNAs were higher. Furthermore, inactivation of the clpP or clpX genes in the gacA mutant restored the stability of RpoS, suggesting that the observed RpoS instability is due to degradation by ClpXP. We also showed that inactivation of rsmA in either the WT or the gacA mutant resulted in RpoS levels higher than in the WT in both stationary and exponential phases, while clpP and clpX transcript levels were significantly reduced. Taken together, these data reveal that in A. vinelandii, the GacA-Rsm system controls RpoS stability through RsmA, which acts as a positive regulator of ClpXP expression.
Rojo-Rodríguez et al. (2026) studied this question.