LAUSR

Gram-negative Escherichia coli is a rod-shaped facultative anaerobic bacterium commonly found in microaerobic/anaerobic environments, including the dental plaques of warm-blooded organisms. These latter can be treated efficiently with fluoride-rich compounds that act as anticaries agents to prevent tooth decay. ABSTRACT Although fluoride-containing compounds are widely used to inhibit bacterial growth, the reprogramming of gene expression underlying cellular responses to fluoride, especially under anaerobic conditions, is still poorly understood. Here, we compare the genome-wide transcriptomic profiles of E. coli grown in the absence (control) or presence (20 and 70 mM) of sodium fluoride (NaF) under anaerobic conditions and assess the impact of fluoride-dependent ATP depletion on RNA turnover. Tiling array analysis revealed transcripts displaying altered abundance in response to NaF treatments. Quantile-based K-means clustering uncovered a subset of genes that were highly upregulated and then downregulated in response to increased and subsequently decreased fluoride concentrations, many of which (~40%) contained repetitive extragenic palindromic (REP) sequences. Northern blot analysis of some of these highly upregulated REP-containing transcripts (i.e., osmC, proP, efeO and yghA) confirmed their considerably enhanced abundance in response to NaF treatment. An mRNA stability analysis of osmC and yghA transcripts demonstrated that fluoride treatment slows down RNA degradation, thereby enhancing RNA stability and steady-state mRNA levels. Moreover, we demonstrate that turnover of these transcripts depends on RNase E activity and RNA degradosome. Thus, we show that NaF exerts significant effects at the whole-transcriptome level under hypoxic growth (i.e., mimicking the host environment), and fluoride can impact gene expression posttranscriptionally by slowing down ATP-dependent degradation of structured RNAs. IMPORTANCE Gram-negative Escherichia coli is a rod-shaped facultative anaerobic bacterium commonly found in microaerobic/anaerobic environments, including the dental plaques of warm-blooded organisms. These latter can be treated efficiently with fluoride-rich compounds that act as anticaries agents to prevent tooth decay. Although fluoride inhibits microbial growth by affecting metabolic pathways, the molecular mechanisms underlying its activity under anaerobic conditions remain poorly defined. Here, using genome-wide transcriptomics, we explore the impact of fluoride treatments on E. coli gene expression under anaerobic conditions. We reveal key gene clusters associated with cellular responses to fluoride and define its ATP-dependent stabilizing effects on transcripts containing repetitive extragenic palindromic sequences. We demonstrate the mechanisms controlling the RNA stability of these REP-containing mRNAs. Thus, fluoride can affect gene expression posttranscriptionally by stabilizing structured RNAs.