role for Lon protease in the control of the acid resistance genes of Escherichia coli

Lon protease is a major protease in cellular protein quality control, but also plays an important regulatory role by degrading various naturally unstable regulators. Here, we traced additional such regulators by identifying regulons with co-ordinately altered expression in a lon mutant by genome-wide transcriptional profiling. Besides many members of the RcsA regulon (which validates our approach as RcsA is a known Lon substrate), many genes of the σS-dependent general stress response were upregulated in the lon mutant. However, the lon mutation did not affect σS levels nor σS activity in general, suggesting specific effects of Lon on secondary regulators involved in the control of subsets of σS-controlled genes. Lon-affected genes also included the major acid resistance genes (gadA, gadBC, gadE, hdeAB and hdeD), which led to the discovery that the essential acid resistance regulator GadE (whose expression is σS-controlled) is degraded in vivo in a Lon-dependent manner. GadE proteolysis is constitutive as it was observed even under conditions that induce the system (i.e. at low pH or during entry into stationary phase). GadE degradation was found to rapidly terminate the acid resistance response upon shift back to neutral pH and to avoid overexpression of acid resistance genes in stationary phase.