AbstractSmall non-coding RNAs play a significant role in regulation of bacterial transcription and translation. Their expression in response to external factors is important for the adaptation of bacteria to changing environmental conditions. We investigated the expression of DrrS, a small noncoding RNA of Mycobacterium tuberculosis, in the mouse model in vivo, in the ex vivo model based upon infected macrophages, and in bacterial cultures, and demonstrated its significant contribution to

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... athogen interactions. Activation of the host immune system triggers NO-inducible up-regulation of DrrS in macrophage-engulfed mycobacteria. Constitutive overexpression of DrrS in cultured mycobacteria launches a broad spectrum of shifts in the bacterial transcriptome profile very similar to those reported for M. tuberculosis adaptation to hostile intra-macrophage environment, and providing defense against oxidative and NO stresses. In addition, we observed dramatic up-regulation of genes for the PE/PPE proteins and proteins of the ESX-1 and ESX-5 secretion systems. Taken together, our results suggest a direct involvement on this small RNA in the interplay between mycobacteria and the host immune system during infectious process.Author summaryPathogenic mycobacteria, including Mycobacterium tuberculosis, are able to survive within host macrophages. In attempt to eliminate intracellular mycobacteria, innate and acquired immune responses of the host activate a number of effector reactions to achieve effective intracellular mycobacterial killing. Mycobacteria, in turn, evolved a plethora of molecular mechanism providing successful escape from host immunity, involving several metabolic pathways allowing transition to dormancy – the state of slow-to-no replicative activity and an increased resistance to external stresses. These mechanisms remain poorly characterized. Small non-coding bacterial RNAs are expressed in response to external factors and play an important role in adaptation of bacteria to changing environmental conditions and escape from host immune responses. We investigated DrrS, a small non-coding RNA of Mycobacterium tuberculosis, in the mouse TB model in vivo, in infected macrophages ex vivo and in bacterial culture, and demonstrated that DrrS up-regulation strictly follows activation of the host immune defense. We established the strain of M. tuberculosis overexpressing DrrS in culture and found that DrrS contributes to mycobacterial resistance to reactive intermediates and activation of dormancy-associated genes, thus participating in bacterial metabolic adaptations and interactions with the host immune system.