Doctor of Philosophy (PhD)
The objective of this dissertation is to understand the functions of Escherichia coli YqjA and YghB, which are members of the conserved DedA/Tvp38 membrane protein family. YqjA and YghB are inner membrane (IM) proteins with multiple predicted membrane–spanning domain, sharing 62% amino acid identity and partly overlapping functions. Simultaneous in-frame deletions of these two genes in a strain named BC202 results in various phenotypes including cell division defects, temperature sensitivity, and sensitivity to drugs and alkaline pH. The cell division defect of BC202 is due to the inefficient secretion of periplasmic amidases by the twin arginine transport (Tat) pathway into the periplasm and drug sensitivity is due to the inefficient function of various drug efflux pumps. Both these phenotypes are related to the loss of proton motive force (PMF) in BC202. Overexpression of MdfA, a Na+-K+/H+ antiporter or growth in acidic media has the ability to rescue all the phenotypes of BC202. In addition, the ∆yqjA mutant (but not the ∆yghB mutant) is alkaline sensitive and overexpression of yqjA can restore growth at alkaline pH only when more than 100mM of sodium or potassium ions is present in the growth medium. Osmotic pressure augments the YqjA mediated growth at alkaline pH. Furthermore, charged amino acids are also essential for YqjA and YghB function that were previously shown important for various secondary transporters. Additionally, yqjA expression is higher at alkaline pH and increased expression of yqjA also required sodium/potassium salts above pH 9.0. The transcriptional regulator CpxR is required for the expression of yqjA at alkaline pH in the presence of Na+/K+. Based on these results, we suggest YqjA and YghB are osmosensing proton-dependent transporters required for E. coli drug resistance and alkaline pH tolerance.
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Kumar, Sujeet, "Roles of members of the conserved DedA/Tvp38 membrane protein family in Escherichia coli drug resistance and alkaline pH tolerance" (2016). LSU Doctoral Dissertations. 3122.