5.2) H-NS, bifunctional RNA and pervasive transcription
A recent study by Zhao et al., (2021) identified the hns mRNA as a bifunctional RNA that down-regulates the expression of its neighbouring gene, galU , thereby attenuating the motility ofS. enterica serovar Typhimurium. Bifunctional (or dual-function) RNAs are defined as RNAs that can both be translated into proteins and play regulatory roles. This group includes mRNAs that overlap with the transcript of neighbouring genes transcribed from the complementary DNA strand (Toledo-Arana and Lasa, 2020). Since these mRNA sequences are complementary, they allow a regulatory mechanism using antisense RNAs. In E. coli and S. enterica , the hns genes are located on the opposite strand to the galU gene (Figure 3b ). The galU gene encodes a uridine triphosphate-glucose-1-phosphate uridyltransferase, an enzyme that facilitates the production of bacterial surface structures such as lipopolysaccharide, extracellular polysaccharide and capsular polysaccharide (Dean and Goldberg, 2002; Guo et al. , 2010; Zhaoet al. , 2021). Zhao et al., (2021) showed that the deletion ofgalU reduced the motility in S. Typhimurium, whereas overexpression of hns mRNA inhibited motility and flagellar gene expression in a galU -related manner. At the molecular level, thegalU mRNA was found to contain an extensive 3’-untranslated region that overlaps with the hns mRNA. This long 3’ untranslated region (UTR) is cleaved, resulting in the production of short RNA fragments. The proposed mechanism involves the formation of an RNA duplex between hns mRNA and the 3’UTR of galU , leading to RNA cleavage and ultimately to galU -processed transcripts that may be translationally inactive (Figure 3b ). Since bifunctional RNAs occur in pairs and have two modes of regulation, it was also suspected that the reverse regulation might also occur. Overexpression of GalU-3’-UTR resulted in a decrease in hns mRNA and protein levels compared to the control strain, leading to the assumption thatgalU mRNA could also act as an antisense RNA and regulatehns mRNA.