## 5 USPs of Pathogenic Bacteria
### Tuberculosis
Experiments using four _usp_ knockout mutants, of which three were DosR-regulated USPs (_Rv1996_, _Rv2005c_ and _Rv2028c_) and the other was the DosR-regulated _Rv2026c USP_, revealed hypoxic survival defects. However, the results seemed to suggest either partial or complete redundancy in USP function in view of the insignificant difference observed in intracellular growth between _usp_ mutants and wild types. Explanations for this unanticipated observation span around the fact that the experiments were conducted _in vitro_, suggesting the repetition of this study in models that are more strongly related to in vivo conditions, as well as the biochemical characterization of USPs to explore their actual functions [25]. In addition, the study attributed the results to the differential regulation and overlapping of USP functions, suggesting their necessity once more than one protein is simultaneously knocked out, possibly owing to them serving various functions during stages of infection. The role or significance of Rv2026c and other USPs specifically in _Mtb_ infection has been investigated by various authors. Studies highlighting Rv2026c along with Rv2421c (probable nicotine-nucleotide adenylyltransferase) as two novel biomarkers for TB diagnosis have shown that among other identified antigens, these two exhibited the best diagnostic performance, as they were both recognized by IgG and IgM antibody responses, with Rv2026c exhibiting considerably higher IgG antibody levels in active TB patients in comparison to healthy controls, while Rv2421c yielded higher levels for both antibody levels in active TB patients [70]. Figure 3: Classification of Mycobacterial USPs. Mycobacterial USPs are grouped into five classes, which altogether contain ten USPs. One of the major concerns of TB is the emergence of resistant strains. Studies have shown that the up-regulation of USPs such as Rv1636, Rv2005c and Rv2623 in _Mtb_ strains resistant to TB secondary defense aminoglycososide drugs such as Kanamycin (KM) and Amikacin (AM) contribute to _Mtb_ resistance. Bioinformatics and proteomic research have shown that Rv2623 over-expresses in AK and KM resistant _Mtb_ strains [73]. The Mycobacterium genome codes for both membrane and secretory proteins. Rv2623 is a universal stress protein homolog categorized as the latter that is up-regulated when the pathogen is confronted by inadequate oxygen levels and nitrosative stress, both conditions being initiators of dormancy. Among all the USPs expressed in mycobacteria, Rv2623 elicits most interest because it is one of the most frequently induced stress proteins during hypoxic and nitric oxide (NO) exposure in the dormancy regulon [74]. Mutants deficient in _Rv2623_ fail to develop chronic lasting infection in mice, while the overexpression of the protein, on the other hand, augments mycobacterial growth, which indicates the importance of the protein establishing both in vivo and in vitro persistent infection. Mycobacterium growth regulation is ATP-dependent both in vivo and _in vitro_, as the protein contains two USP domains in tandem that are connected by anti-parallel sheets, which altogether possess four ATP-bound nucleotide-binding pockets [74; 75]. In conjunction with this finding, an increase in the protein was observed after macrophage phagocytosis as well as in the lungs of infected mice [76; 77; 78]. More in-depth studies have shown that Rv2623 negatively regulates mycobacterial growth by interacting with Rv1747, a putative ATP binding cassette transporter that exports lipopolysaccharide. Phosphorylation of Thr237 of Rv2623 in the presence of certain signals in the host causes the formation of a phosphothreonine-containing motif, which facilitates interaction between the FHA I domain of Rv1747 and the Rv2623 USP (Figure 4) [79]. This interaction; however, turns Rv1747 off.