The two-component signal transduction system (TCS) BarA/UvrY activates transcription of CsrB and CsrC noncoding RNAs, which act by sequestering the RNA-binding global regulatory protein CsrA. Here, we show that the metabolic end products formate and acetate provide a physiological stimulus for this TCS and thus link posttranscriptional regulation by the Csr system to the metabolic state of the cell.