Riboregulation by DsrA RNA: trans-actions for global economy.

DsrA is an 87 nucleotide Escherichia coli RNA with extraordinary regulatory properties. The profound impact of its actions stems from DsrA regulating translation of two global transcription regulators, H-NS and RpoS (sigmas), by sequence-specific RNA-RNA interactions. H-NS is a major nucleoid-structuring and global repressor protein, and RpoS is the stationary phase and stress response sigma factor of RNA polymerase. DsrA changes its conformation to bind to these two different mRNA targets and thereby inhibits H-NS translation, while stimulating that of RpoS in a mechanistically distinct fashion. DsrA apparently binds both the start and the stop codons of hns mRNA and sharply decreases the mRNA half-life. DsrA also binds sequences in the 5'-untranslated leader region of rpoS mRNA, enhancing rpoS mRNA stability and RpoS translation. A cohort of genes, governed by H-NS repression and RpoS activation, are thus regulated. Low temperatures increase the levels of DsrA, with differential effects on H-NS and RpoS. Additionally, the RNA chaperone protein Hfq is involved with DsrA regulation, as well as with other small RNAs that also act on RpoS to co-ordinate stress responses. We address the possible functions of this genetic regulatory mechanism, as well as the advantages of using small RNAs as global regulators to orchestrate gene expression.