## 3 Results and Discussion
### LgAC is an Optogenetic Modulator of cAMP in E. coli
#### 3.2.3 Optogenetic Modulation of cAMP Regulates Virulence Signaling in _E. coli_  
The involvement of cAMP in the regulation of bacterial virulence is a well-known phenomenon [60]. In _Mycobacterium tuberculosis_, 10 functional adenylate cyclases (ACs) and CRP were reported to be involved in the complex regulation of virulence [61]. Quantitative proteomics showed a reduced expression of proteins (ClpP, YebC, KatG, BtuE, and Zur) associated with virulence in _E. coli_ after BL illumination. The caseinolytic protease P (ClpP), a serine protease, was significantly reduced (~2 fold) in the LgAC-expressing BL-illuminated _E. coli_ (Figure 2B; Table 1). In bacterial pathogens, ClpP plays an important role in the onset of pathogenesis and virulence [62]. The Clp mutant of _Salmonella typhimurium_ was unable to grow in peritoneal macrophages, which is a crucial step for the establishment of _S. typhimurium_ virulence [63]. [PERSON] and coworkers demonstrated that _S. typhimurium_ virulence was attenuated in the Clp mutant due to the mis-regulation of RpoS as well as the indirect regulation of CsrA and the genes of the _Salmonella_ pathogenicity island 1 [64]. YebC, a PmpR (_pqsR_-mediated POS regulator) family protein, which regulates quorum sensing in the bacterial system [65], was significantly downregulated (~1.5 fold) in _E. coli_ after BL illumination (Figure 2B; Table 1). Bacterial virulence depends on the type III secretion system (T3 SS), type VI secretion system (T6 SS), and quorum sensing system (QS), which controls cell-to-cell communication and helps the bacterial population to invade and interact with hosts [66; 67; 68]. [PERSON] et al. [49; 69] also reported the role of the cAMP-CRP complex in the regulation of HapR expression (a major regulator of quorum sensing) in a cell density-dependent manner in _V. cholerae_ via control of the Fis and RpoS pathways. Therefore, modulating cAMP-dependent quorum sensing may control the bacterial pathogenesis and compromise host-pathogen interactions.  
Bacterial pathogenesis elicits a host immune response to generate and release toxic reactive oxygen species (ROS) by phagocyte cells. However, many pathogens have evolved an arsenal of effective oxidative stress defense systems to cope with such conditions, such as molecular scavengers,proteins, and DNA repair enzymes [70]. Quantitative proteomics in the current study revealed the downregulation of proteins associated with oxidative stress in the LgAC-expressing _E. coli_ after BL illumination. An oxidative stress defense protein was significantly downregulated (\(\sim\)1.5 fold) in _E. coli_ after BL illumination. Furthermore, a \(\sim\)2-fold downregulation of BtuE protein (glutathione peroxidase) and specific catalase (KatE) was also observed (Figure 2B; Table 1). In _E. coli_, BtuE has non-specific peroxide activity, and it utilizes thioredoxin A for the decomposition of different hydroperoxides [71]. In _Xanthomonas axonopodis_ pv. _citris_, KatE is required for full virulence in Citris plant, where KatE is involved in the colonization and survival of _X. axonopodis_ during pathogenesis [72]. Significant downregulation (\(\sim\)4 fold) of the Zur protein (zinc uptake regulator) was observed in _E. coli_ after BL illumination (Figure 2B; Table 1). Nutritional immunity is one of the most common responses posed by the host upon pathogen invasion, wherein the host intoxates the invading microbes with essential metals, of which zinc is an important component. Several microbes have evolved ways to adapt hypozincemic or hyperzincmic conditions [73]. In bacteria, Zur regulated the expression of genes involved in the control of zinc homeostasis and the onset of oxidative stress [74].