Cpx-dependent expression of YqjA requires cations at elevated pH.

Under alkaline pH conditions, Escherichia coli must maintain a stable cytoplasmic pH of about 7.6 that is acidic relative to the environment. Bacteria employ various mechanisms to survive alkaline pH; however, membrane cation/H+ antiporters play a primary role by facilitating inward transport of protons. Escherichia coli YqjA belongs to the DedA/Tvp38 membrane protein family and, along with its paralog YghB, is required for growth at 42°C, proper cell division and antibiotic resistance. YqjA is required for viability at alkaline pH, requiring cations sodium or potassium to support growth under these conditions, suggesting it may be a transporter. We measured yqjA expression at different pHs and cation concentrations using a yqjA promoter-lacZ fusion. We found that yqjA promoter activity was highest at alkaline pH. Increased activity of the yqjA promoter required both the transcriptional regulator CpxR, in agreement with previous results, and sodium or potassium salts at alkaline pH. Extracellular cations are also required for activity of cpxP and degP promoters at alkaline pH, suggesting this is a general property of the Cpx regulon. To our knowledge, this is the first demonstration of cation-dependent expression of Cpx-regulated genes at alkaline pH.