## 2 sRNA as Orchestrators of Bacterial Pathogenesis
### Evading Immune Response
#### 2.3.2 MTS1338  
In Mtb, sRNA MTS1338 plays a crucial role in virulence and intracellular survival [39; 120]. MTS1338, highly conserved across pathogenic mycobacteria, is upregulated in dormant Mtb and macrophage-phagocytosed cells, suggesting a role in host-pathogen interactions. Elevated MTS1338 expression during infection is linked to transcriptional changes indicative of reduced metabolism and cell division, strategies associated with stress survival. Transcriptomic analyses revealed a characteristic response to MTS1338 overexpression during logarithmic and stationary phases, with increased resistance to low pH, reactive oxygen species (ROS), and reactive nitrogen species (RNS) in logarithmic-phase [121]. Survival analysis demonstrated enhanced stress resistance, highlighting MTS1338's role in adapting to the macrophage environment. Interestingly, approximately 50% of differentially expressed genes (DEGs) in MTS1338-overexpressing strains overlapped with those activated in Mtb residing within macrophages, underscoring its importance in intracellular adaptation. These include MPT70, encoding an antigen involved in T-cell differentiation and expressed in later infection stages, Rv2034 and Rv2035, components of a toxin-antitoxin system and regulators of virulence and stress responses, and CmtR (Rv1994c) and Rv2642, ArsR-family regulators linked to ROS detoxification and metal stress (Figure 1e).  
MTS1338 is regulated by DosR, a transcriptional activator under hypoxic conditions, although it is not fully dependent on DosR [122; 123]. The DevR/DosR response regulator plays a crucial role in the virulence, dormancy adaptation, and antibiotic tolerance of Mtb by regulating the dormancy regulon. Previously, we developed recombinant tools to study gene regulation in Mtb [124; 125]. Using this in vivo reporter assay, we characterized key residues involved in DevR-RNA polymerase interactions. In short, this system utilizes _E. coli_ co-transformed with plasmids expressing Mtb RNA polymerase subunits (\(\alpha\), \(\beta\), \(\beta^{\prime}\), and \(\sigma^{\Delta}\)), a DevR expression cassette, and a GFP reporter under the DevR-regulated _fdxA_ promoter. We found that the amino acid E280 in the \(\alpha\)-subunit of RNAP and K513 and R515 in sigma factor A are involved in DevR-mediated transcription. In silico modeling suggests that interactions between key residues of DevR (D172, E178) and sigma factor A (Q505, R515, K513) are required for DevR-mediated transcriptional regulation [126]. MTS1338 upregulation has been observed in hypoxic, acidic, and oxidative stress conditions, suggesting a role in dormancy [120]. Biochemical studies indicate that DosR binds with high affinity to MTS1338's promoter region, enhancing its expression [127]. Although PhoP does not directly bind to the MTS1338 promoter, it interacts with the DosR-MTS1338 complex, indicating a coordinated regulation mechanism. Taken together, these findings underscore MTS1338's importance in Mtb pathogenicity, facilitating adaptation to macrophage-induced stress and promoting bacterial survival during chronic infection.  
It is thus clear that bacteria have evolved a diverse repertoire of sRNAs to regulate virulence factor expression, facilitating host infection and immune evasion. Beyond virulence regulation, sRNAs also contribute to complex stress response pathways, enabling bacterial adaptation to various host-induced stresses. The following section explores the role of sRNAs in mediating stress responses to counteract host defense mechanisms.