In vivo and in vitro analysis of 6S RNA-templated short transcripts in Bacillus subtilis

By differential high-throughput RNA sequencing (dRNA-seq) we have identified "product RNAs" (pRNAs) as short as 8-12 nucleotides that are synthesized by Bacillus subtilis RNA polymerase (RNAP) in vivo using the regulatory 6S-1 RNA as template. The dRNA-seq data were confirmed by in vitro transcription experiments and Northern blotting. In our libraries, we were unable to detect statistically meaningful numbers of reads potentially representing pRNAs derived from 6S-2 RNA. However, pRNAs could be synthesized in vitro from 6S-2 RNA as template by the B. subtilis σ(A) RNAP. 6S-1 pRNA levels are low during exponential, increase in stationary, and burst during outgrowth from stationary phase, demonstrating that pRNA synthesis is a conserved regulatory mechanism, but a more dynamic and fine-tuning process than previously thought. Most pRNAs have a length of 8-15 nt, very few up to 24 nt. The average length of pRNAs tended to increase from stationary to outgrowth conditions. Synthesis of pRNA is initiated at C40 of 6S-1 RNA and U41 of 6S-2 RNA, yielding pRNAs with a 5'-terminal G or A residue, respectively. A B. subtilis 6S-1 RNA mutant strain encoding a pRNA with a 5'-terminal A residue showed the same relative distribution of ~14-nt pRNAs between the different growth states, but generally displayed lower pRNA levels than the reference strain encoding wild-type 6S-1 RNA. A ~two-fold lower affinity of the C40U mutant 6S-1 RNA towards σ(A) RNAP may have contributed to this reduction in pRNA levels. We infer that 6S-1 pRNA synthesis, although evolutionarily optimized for initiation with a +1G residue, is not primarily regulated at the transcription initiation level via growth phase-dependent variations in the cellular GTP pool.