How mycobacterial RNase E cuts RNA: new rules emerge

RNA levels inside bacterial cells change quickly as microbes respond to stress, and controlling those RNA pools is especially important for pathogens such as Mycobacterium tuberculosis. Much of that control comes from RNA degradation, and a single enzyme, RNase E, plays a rate-limiting role in breaking down the majority of mycobacterial transcripts. Despite this central role, scientists still lack clear rules about what RNAs RNase E will cut efficiently. In work led by Scarlet S. Shell, researchers set out to define the basic substrate requirements and cleavage preferences of the mycobacterial enzyme. Rather than inferring activity from complex cell extracts, they examined the enzyme’s behavior directly in vitro, using RNase E from both Mycolicibacterium smegmatis and M. tuberculosis. By testing many different RNA pieces under controlled conditions, the team asked how length, chemical groups at the RNA ends, and sequence features influence where and how RNase E cuts. Their approach focused on isolating the enzyme’s intrinsic preferences so that the community would have clearer, experimentally grounded rules for how RNase E contributes to RNA turnover in mycobacteria.

The central experimental approach was in vitro cleavage assays using purified RNase E from both Mycolicibacterium smegmatis and M. tuberculosis. These experiments revealed a strict minimum size requirement: RNase E was only active on substrates with a minimum length of approximately 27 nt. The researchers also tested the chemical state of the RNA 5’ end and found a clear preference for substrates bearing 5’ monophosphates rather than 5’ triphosphates. Mapping where cuts occurred within longer RNAs showed that cleavage positions are dictated by a combination of the RNA sequence itself and the distance from RNA ends — both factors influenced where RNase E made scissions. The team observed patterns suggesting that the accumulation of cleavage products might feed back on the enzyme, consistent with product inhibition. Finally, parallel tests with M. smegmatis RNase E produced results that closely matched those from M. tuberculosis RNase E, supporting the use of the nonpathogenic species as a model for studying mycobacterial RNase E in vitro.

These findings sharpen our picture of how RNase E shapes RNA levels in mycobacteria. Knowing that RNase E requires about 27 nt and that it favors 5’ monophosphates gives researchers concrete rules to predict which transcripts are likely to be processed or degraded by the enzyme. The dual dependence of cleavage sites on sequence and distance from RNA ends helps explain why some RNA regions are more stable than others, and the indication of possible product inhibition suggests that RNase E activity can be self-limiting under certain conditions. Importantly, showing that M. smegmatis RNase E behaves like M. tuberculosis RNase E validates a safer, faster model organism for detailed biochemical studies. Together, these results provide a clearer experimental foundation for future work on RNA stability, gene regulation, and stress responses in mycobacteria, and they will help investigators design experiments and interpret RNA decay data with greater confidence.