The transcription of antisense RNAs (asRNAs) has been shown to be widespread in bacterial genomes, but whether these asRNAs have a regulatory role like other non-coding RNAs has been unclear. Ochman and colleagues postulated that functional elements are likely to be conserved between genomes, and thus compared the genome-wide expression of asRNAs in two enteric bacteria, Escherichia coli and Salmonella enterica subsp. enterica serovar Typhimurium. asRNAs were highly expressed in both organisms and were mostly expressed under the control of the σ transcription factor. However, fewer than 3% of asRNAs were highly expressed by both bacteria. Moreover, unlike the situation with mRNA promoters, there was no evidence of sequence conservation in asRNA promoters between E. coli and S. Typhimurium or even between two Escherichia spp. or two strains of E. coli. Thus, the authors concluded that most asRNAs are not likely to be functional.