Feedback regulation of RNase E during UV-stress response in the cyanobacterium Synechocystis sp. PCC 6803

Endoribonucleases govern the maturation and degradation of RNA and are indispensable in the posttranscriptional regulation of gene expression. A key endoribonuclease in many bacteria is RNase E. To ensure an appropriate supply of RNase E, some bacteria, such as E. coli, have evolved tightly functioning feedback regulation of RNase E that is mediated in cis by the rne 5'-untranslated region (5' UTR); however, the mechanisms involved in the control of RNase E in other bacteria largely remain unknown. Cyanobacteria rely on solar light as an energy source for photosynthesis, despite the inherent ultraviolet (UV) irradiation. Here, we investigated the global gene expression response in the cyanobacterium Synechocystis sp. PCC 6803 after exposure to UV light and discovered a unique response of RNase E: a rapidly increasing enzymatic activity, although the stability of the protein was decreased. In parallel, we observed an increased accumulation of full-length rne mRNA that was caused by the stabilization of its 5' UTR and suppression of premature transcriptional termination but not by an increased transcription rate. Mapping of RNA 3' ends and in vitro cleavage assays revealed that RNase E cleaves within a stretch of six consecutive uridine residues within the rne 5' UTR, indicating autoregulation via its own 5' UTR. These observations imply that RNase E in cyanobacteria contributes substantially to reshaping the transcriptome during the UV stress response and that its required activity level is maintained despite enhanced turnover of the protein by posttranscriptional feedback regulation.