CpxR-dependent thermoregulation of PrtA metalloprotease expression and its contribution to bacterial biofilm formation.

PrtA is the major secreted metalloprotease of Serratia marcescens. Previous reports implicate PrtA in the pathogenic capacity of this bacterium. PrtA is also clinically used as a potent analgesic and anti-inflammatory drug, and its catalytic properties attract industrial interest. Comparatively, there is scarce knowledge about the mechanisms that physiologically govern PrtA expression in Serratia. In this work, we demonstrate that PrtA production is derepressed when the bacterial growth temperature decreases from 37 degrees C to 30 degrees C. We show that this thermoregulation occurs at the transcriptional level. We determined that upstream of prtA, there is a conserved motif that is directly recognized by the CpxR transcriptional regulator. This feature is found along Serratia strains irrespective of their isolation source, suggesting an evolutionary conservation of CpxR-dependent regulation of PrtA expression. We found that in S. marcescens, the CpxAR system is more active at 37 degrees C than at 30 degrees C. In good agreement with these results, in a cpxR mutant background, prtA is derepressed at 37 degrees C, while overexpression of the NlpE lipoprotein, a well-known CpxAR-inducing condition, inhibits PrtA expression, suggesting that the levels of the activated form of CpxR are increased at 37 degrees C over those at 30 degrees C. In addition, we establish that PrtA is involved in the ability of S. marcescens to develop biofilm. In accordance, CpxR influences the biofilm phenotype only when bacteria are grown at 37 degrees C. In sum, our findings shed light on regulatory mechanisms that fine-tune PrtA expression and reveal a novel role for PrtA in the lifestyle of S. marcescens. IMPORTANCE We demonstrate that S. marcescens metalloprotease PrtA expression is transcriptionally thermoregulated. While strongly activated below 30 degrees C, its expression is downregulated at 37 degrees C. We found that in S. marcescens, the CpxAR signal transduction system, which responds to envelope stress and bacterial surface adhesion, is activated at 37 degrees C and able to downregulate PrtA expression by direct interaction of CpxR with a binding motif located upstream of the prtA gene. Moreover, we reveal that PrtA expression favors the ability of S. marcescens to develop biofilm, irrespective of the bacterial growth temperature. In this context, thermoregulation along with a highly conserved CpxR-dependent modulation mechanism gives clues about the relevance of PrtA as a factor implicated in the persistence of S. marcescens on abiotic surfaces and in bacterial host colonization capacity.