Effects of Amino Acid Supplementation on Porin Expression and ToxR Levels in Vibrio cholerae

Abstract

ABSTRACTVibrio choleraeresponds to environmental changes by altering the protein composition of its outer membrane. In rich medium,V. choleraeexpresses almost exclusively the outer membrane porin OmpU, whereas in minimal medium, OmpT is the dominant porin. The supplementation of a minimal medium with a mixture of asparagine, arginine, glutamic acid, and serine (NRES) promotes OmpU production and OmpT repression at levels similar to those seen with rich media. Here we show that the altered Omp profile is not due to an increase in the growth rate in the presence of supplemental amino acids but requires the addition of specific amino acids. The effects of the NRES mix on Omp production were mediated by ToxR, a known regulator ofompgene expression. No changes in the Omp profile were detected in atoxRmutant. Supplementation with the NRES mix resulted in significantly higher levels of ToxR, and the elevated ToxR levels were sufficient to cause a switch in Omp synthesis. The increase in the level of the ToxR protein correlated with an increase intoxRmRNA levels and was observed only whentoxRwas expressed from its native promoter. ToxS, which is required for ToxR activity, was necessary for NRES-mediatedompgene regulation but not for the increase in ToxR levels. The growth ofV. choleraein the presence of bile acids also resulted in Omp switching, and this required ToxR. However, unlike the NRES mix, bile acids did not increase either ToxR protein ortoxRmRNA levels, suggesting a different mechanism ofompgene regulation by bile than that by amino acids.