Identification and characterization of a cis antisense RNA of the parC gene encoding DNA topoisomerase IV of Salmonella enterica serovar Typhi

Bacterial cis-encoded antisense RNAs are transcribed from the opposite strand of protein coding genes, and their regulatory roles adapt cells to changing environmental conditions. By deep sequencing of the transcriptome of Salmonella enterica serovar Typhi, an antisense RNA that is encoded in cis to the parC gene was found. parC encodes the subunit A component of topoisomerase IV, a class of enzymes that relax both positively and negatively supercoiled DNA and are also required for segregation of daughter chromosomes in bacteria. Transcription of the 871 nucleotide antisense RNA was confirmed by northern blot and RACE analysis to be expressed mostly in the stationary phase of bacterial growth and also upregulated in iron limitation and osmotic stress conditions. Overexpression of the antisense RNA resulted in a significant increase in parC mRNA levels. Further analysis revealed that expression of the antisense RNA stabilizes the target mRNA, probably by protecting it from endoribonucleases. Our findings confirm and add to the ever increasing knowledge of the important role that regulatory antisense RNAs play in bacteria.