Escherichia coli responds to external acidification (pH 4.0 to 5.0) by synthesizing a newly identified, ∼450-nucleotide RNA component. At maximal levels of induction it is one of the most abundant small RNAs in the cell and is relatively stable bacterial RNA. The acid-inducible RNA was purified, and the gene encoding it, designated asr (for acid shock RNA), mapped at 35.98 min on the E. coli chromosome. Analysis of the asr DNA sequence revealed an open reading frame coding for a 111-amino-acid polypeptide with a deduced molecular mass of approximately 11.6 kDa. According to computer-assisted analysis, the predicted polypeptide contains a typical signal sequence of 30 amino acids and might represent either a periplasmic or an outer membrane protein. Theasr gene cloned downstream from a T7 promoter was translated in vivo after transcription using a T7 RNA polymerase transcription system. Expression of a plasmid-encodedasr::lacZ fusion under a nativeasr promoter was reduced ∼15-fold in a complex medium, such as Luria-Bertani medium, versus the minimal medium. Transcription of the chromosomal asr was abolished in the presence of aphoB-phoR (a two-component regulatory system, controlling the pho regulon inducible by phosphate starvation) deletion mutant. Acid-mediated induction of the asr gene in the Δ(phoB-phoR) mutant strain was restored by introduction of the plasmid with cloned phoB-phoR genes. Primer extension analysis of the asr transcript revealed a region similar to the Pho box (the consensus sequence found in promoters transcriptionally activated by the PhoB protein) upstream from the determined transcription start. The asr promoter DNA region was demonstrated to bind PhoB protein in vitro. We discuss our results in terms of how bacteria might employ the phoB-phoRregulatory system to sense an external acidity and regulate transcription of the asr gene.