Thermo-Regulation of the cmaX-cfrX-cmpX Operon IMPORTANCE Because temperature is one of the first physical signal that allows bacteria to distinguish a potential host from its environment, the response of P. aeruginosa has been more investigated in case of elevated rather than low temperature. Here, we show that the ECFσ SigX is an effector in the latter condition, which controls the transcription of the two members of an intriguing operon encoding the mechanosensitive channel CmpX and the hypothetical protein CfrX, while cmaX is under the control of AlgU. In addition, we show that SigX is activated following membrane structure alterations. More generally, we have identified temperature as a key environmental factor regulating antagonistically the expression and the activity of the two major cell wall stress response ECFσ in P. aeruginosa, suggesting that they are involved in specific but interconnected networks, in which the AlgU-dependent global transcriptional regulator AmrZ (alginate and motility regulator Z) could play a role by a mechanism that remains to be explored.