E. coli 6S RNA complexed to RNA polymerase maintains product RNA synthesis at low cellular ATP levels by initiation with noncanonical initiator nucleotides.

The E. coli 6S RNA is an RNA polymerase (RNAP) inhibitor that competes with σ70-dependent DNA promoters for binding to RNAP holoenzyme (RNAP:σ70). The 6S RNA when bound is then used as a template to synthesize a short product RNA (pRNA; usually 13-nt-long). This pRNA changes the 6S RNA structure, triggering the 6S RNA:pRNA complex to release and allowing DNA-dependent housekeeping gene expression to resume. In high nutrient conditions, 6S RNA turnover is extremely rapid but becomes very slow in low nutrient environments. This leads to a large accumulation of inhibited RNAP:σ70 in stationary phase. As pRNA initiates synthesis with ATP, we and others have proposed that the 6S RNA release rate strongly depends on ATP levels as a proxy for sensing the cellular metabolic state. By purifying endogenous 6S RNA:pRNA complexes using RNA Mango and using reverse transcriptase to generate pRNA-cDNA chimeras, we demonstrate that 6S RNA:pRNA formation can be simultaneous with 6S RNA 5' maturation. More importantly, we find a dramatic accumulation of capped pRNAs during stationary phase. This indicates that ATP levels in stationary phase are low enough for noncanonical initiator nucleotides (NCINs) such as NAD+ and NADH to initiate pRNA synthesis. In vitro, mutation of the conserved 6S RNA template sequence immediately upstream of the pRNA transcriptional start site can increase or decrease the pRNA capping efficiency, suggesting that evolution has tuned the biological 6S RNA sequence for an optimal capping rate. NCIN-initiated pRNA synthesis may therefore be essential for cell viability in low nutrient conditions.

Purification of RNA Mango Tagged Native RNA-protein Complexes from Cellular Extracts Using TO1-Desthiobiotin Fluorophore Ligand.

A native purification strategy using RNA Mango for RNA based purification of RNA-protein complexes is described. The RNA Mango aptamer is first genetically engineered into the RNA of interest. RNA Mango containing complexes obtained from cleared cellular native extracts are then immobilized onto TO1-Desthiobiotin saturated streptavidin agarose beads. The beads are washed to remove non-specific complexes and then the RNA Mango containing complexes are eluted by the addition of free biotin to the beads. Since the eluted complexes are native and fluorescent, a second purification step such as size exclusion chromatography can easily be added and the purified complexes tracked by monitoring fluorescence. The high purity native complexes resulting from this two-step purification strategy can be then used for further biochemical characterization.