## 3 Results and Discussion
### LgAC is an Optogenetic Modulator of cAMP in E. coli
#### 3.2.1. _LgAC_ Regulates Growth and Energy Metabolism in _E. coli_  
The KEGG pathway analysis suggested that most DEPs are involved in growth and energy metabolism (Figure 2C; Table 2). Proteomic analysis showed a \(\sim\)2-fold reduction in the expression of growth-related proteins, i.e., phosphoenolpyruvate-dependent phosphotransferase (PTS), adenylate kinase (Adk), and acetate kinase (AckA) in recombinant _E. coli_ illuminated with BL (Figure 2B; Table 1). In _E. coli_, the cyclic AMP (cAMP)-catabolite receptor protein (CRP) complex regulates (positively or negatively) the catabolic repression and transcription of several genes involved in carbohydrate catabolism [2; 32]. The inactivation of either adenylate cyclase or CRP would lead to highly pleiotropic phenotypes [33]. [PERSON] and coworkers showed the correlation between the intracellular cAMP concentration and growth in _E. coli_[33]. Phosphoenolpyruvate-dependent phosphotransferase (PTS) regulates carbon catabolism via phosphoenolpyruvate-dependent protein kinase enzyme IIACrr (EIIACrr)-mediated inducer exclusion and cAMP-CRP-based catabolic repression in _E. coli_. A reduction in PTS expression is explained by the fact that phosphorylated EIIACrr activates bacterial adenylate cyclase, which further synthesizes cAMP [33]. However, in the present experiment, adenylate cyclase activation was light induced, which bypasses the requirement of EIIACrr. Adenylate kinase is another crucial enzyme involved in regulating the growth of _E. coli_ by controlling the cellular ATP levels [34]. In _Streptococcus pneumoniae_, the adenylate kinase (SpAdk) level and growth rate were correlated, where mutation in SpAdk caused growth defects in vivo [35]. The ratio of phosphorylated and non-phosphorylated AckA on the other hand regulates the phosphoenzyme I/enzyme I ratio of PTS as well as the ATP/ADP ratio, respectively. This in turn controls the rate of sugar transport and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**KEGG TERM** & **KEGG ID** & **Input Number** & \(\boldsymbol{p}\)**-Value** \\ \hline Carbon metabolism & eco01200 & 14 & \(1.76\times 10^{-13}\) \\ Microbial metabolism in diverse environments & eco01120 & 14 & \(4.91\times 10^{-9}\) \\ Biosynthesis of secondary metabolites & eco01110 & 14 & \(6.47\times 10^{-8}\) \\ Pyruvate metabolism & eco00620 & 6 & \(5.29\times 10^{-6}\) \\ Biosynthesis of antibiotics & eco01130 & 10 & \(5.58\times 10^{-6}\) \\ Metabolic pathways & eco01100 & 17 & \(1.61\times 10^{-5}\) \\ Glyoxylate and dicarboxylate metabolism & eco00630 & 5 & \(2.99\times 10^{-5}\) \\ Citrate cycle (TCA cycle) & eco00020 & 4 & \(0.000102\) \\ Glyolysis / Glucoseogenesis & eco00010 & 4 & \(0.000487\) \\ Methane metabolism & eco00680 & 3 & \(0.001768\) \\ Pentose phosphate pathway & eco00030 & 3 & \(0.002336\) \\ Biosynthesis of amino acids & eco01230 & 5 & \(0.002993\) \\ Ubiquitinone and other terpenoid-quinone biosynthesis & eco00130 & 2 & \(0.013632\) \\ Butanoate metabolism & eco00650 & 2 & \(0.040092\) \\ Propanoate metabolism & eco00640 & 2 & \(0.041988\) \\ Fructose and mannose metabolism & eco00051 & 2 & \(0.047874\) \\ Oxidative phosphorylation & eco00190 & 2 & \(0.047874\) \\ Tryptophan metabolism & eco00380 & 1 & \(0.079139\) \\ Two-component system & eco02020 & 3 & \(0.127058\) \\ Argin biosynthesis & eco00220 & 1 & \(0.145133\) \\ Glutathione metabolism & eco00480 & 1 & \(0.152176\) \\
2-Oxcarboxylic acid metabolism & eco01210 & 1 & \(0.199917\) \\ Nitrogen metabolism & eco00910 & 1 & \(0.199917\) \\ Alanine, aspartate and glutamate metabolism & eco00250 & 1 & \(0.232407\) \\ Cysteine and methionine metabolism & eco00270 & 1 & \(0.238749\) \\ Sulfur metabolism & eco00920 & 1 & \(0.245041\) \\ Glycine, serine and threonine metabolism & eco00260 & 1 & \(0.263613\) \\ Quorum sensing & eco02024 & 1 & \(0.386875\) \\ ABC transporters & eco02010 & 1 & \(0.766284\) \\ \hline \hline \end{tabular}
\end{table}