Abstract
Altering membrane protein and lipid composition is an important strategy for maintaining membrane integrity during environmental stress. Many bacterial small RNAs (sRNAs) control membrane protein production, but sRNA-mediated regulation of membrane fatty acid composition is less well understood. The sRNA RydC was previously shown to stabilize cfa (cyclopropane fatty acid synthase) mRNA, resulting in higher levels of cyclopropane fatty acids in the cell membrane. Here, we report that additional sRNAs, ArrS and CpxQ, also directly regulate cfa posttranscriptionally. RydC and ArrS act through masking an RNase E cleavage site in the cfa mRNA 5' untranslated region (UTR), and both sRNAs posttranscriptionally activate cfa In contrast, CpxQ binds to a different site in the cfa mRNA 5' UTR and represses cfa expression. Alteration of membrane lipid composition is a key mechanism for bacteria to survive low-pH environments, and we show that cfa translation increases in an sRNA-dependent manner when cells are subjected to mild acid stress. This work suggests an important role for sRNAs in the acid stress response through regulation of cfa mRNA.IMPORTANCE Small RNAs (sRNAs) in bacteria are abundant and play important roles in posttranscriptional regulation of gene expression, particularly under stress conditions. Some mRNAs are targets for regulation by multiple sRNAs, each responding to different environmental signals. Uncovering the regulatory mechanisms governing sRNA-mRNA interactions and the relevant conditions for these interactions is an ongoing challenge. In this study, we discovered that multiple sRNAs control membrane lipid composition by regulating stability of a single mRNA target. The sRNA-dependent regulation occurred in response to changing pH and was important for cell viability under acid stress conditions. This work reveals yet another aspect of bacterial physiology controlled at the posttranscriptional level by sRNA regulators.
Highlights
Altering membrane protein and lipid composition is an important strategy for maintaining membrane integrity during environmental stress
Transcription of cfa is controlled by two promoters that yield mRNAs with a 212-nt 5= untranslated region (UTR) or a 34-nt 5= UTR (S-dependent promoter) [8] (Fig. 1A)
The long mRNA isoform is subject to posttranscriptional regulation by the small RNAs (sRNAs) RydC, which stabilizes the transcript by base-pairing at an RNase E recognition site within the 5= UTR [11]
Summary
Altering membrane protein and lipid composition is an important strategy for maintaining membrane integrity during environmental stress. Many bacterial small RNAs (sRNAs) control membrane protein production, but sRNA-mediated regulation of membrane fatty acid composition is less well understood. Alteration of membrane lipid composition is a key mechanism for bacteria to survive low-pH environments, and we show that cfa translation increases in an sRNA-dependent manner when cells are subjected to mild acid stress. This work suggests an important role for sRNAs in the acid stress response through regulation of cfa mRNA. We discovered that multiple sRNAs control membrane lipid composition by regulating stability of a single mRNA target. The sRNAdependent regulation occurred in response to changing pH and was important for cell viability under acid stress conditions. While de novo production of fatty acids is an important adaptation mechanism, bacteria may encounter abrupt environmental changes that require rapid modification of the fatty acids already incorporated in the membrane. The 70-dependent promoter is functional throughout growth, whereas transcription from the s promoter occurs only during stationary phase
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