## 1 Introduction  
Cyclic adenosine monophosphate (cAMP) is a secondary messenger, which plays a central role in a myriad of signaling processes and is synthesized by a specific membrane or soluble protein, adenylate cyclase. This secondary messenger coordinates complex signaling by controlling intracellular metabolites like fructose 1,6-bisphosphate, phosphoenolpyruvate, acetyl-Coenzyme A, \(\alpha\)-ketoglut acid, pyruvate, and oxaloacetate, known to be essential for enzyme-level regulation, transcriptional control, and nutrient homeostasis [1, 2]. In bacteria, cAMP directly binds and activates the transcription factor of the cAMP receptor protein (CRP) family [3]. In _Escherichia coli_, cAMP is primarily known to be involved in mediating the glucose response or catabolite repression [4], but its role has now been expanding into the domains of biofilm formation, microbial virulence, and pathogenesis [3]. The cAMP-CRP complex controls extracellular matrix production and biofilm formation through transcriptional regulation of the _csg_D gene [5]. [PERSON] and coworker showed that cAMP may act as a signaling molecule, which drives the uropathogenic _E. coli_ invasion from the bladder lumen to the interior of epithelial cells [5]. [PERSON] et al. [6] suggested that the metabolic sensor cAMP-CRP regulates type 1 fimbiration, which is required for attachment and colonization of the uroepithelium. The surface attachment and biofilm formation are key factors for the onset of microbial pathogenesis and are also the major contributors to human diseases. The biofilm growth of _E. coli_ might be a cAMP-dependent (mediated by phosphoenolpyruvate-dependent phosphotransferase, PTS) or -independent (pH-dependent) process [7]. Recently, [PERSON] et al. [8] also demonstrated the association of cAMP-CRP complex with virulence, metal acquisition, and quorum sensing in addition to carbon metabolism. Several crucial bacterial processes may be managed by the modulation of the intracellular cAMP concentration, which might play an important role in the development of novel antibacterial drugs. Researchers have tried to modulate the cAMP level in cellular systems to regulate the cyclic nucleotide(s)-mediated signaling in a controlled manner. Agonists activating adenylate cyclases [9], and phosphodiesterase (PDE) inhibitors [10], have been employed to modulate cAMP-dependent signaling; however, these methods lack spatial and temporal resolution. It was suggested that genetically encoded photo-switchable cyclases would provide scientists an opportunity to achieve cell-targeted control of cAMP levels by illumination [11]. In early 2005, channel-hodopsin (ChR) and other rhodopsins were used as genetically encoded photo-switches, for controlling the neural activity simply by illumination and such applications have paved the way for the development of the field of optogenetics [12].The successful application of these light-sensitive proteins as an optogenetic tool has encouraged scientists to identify and apply photoreceptor proteins for light-dependent regulation of cyclic nucleotide(s), protein-protein interaction, and protein translocation events in a cell. The photoactivated adenylate cyclases (PACs) were genetically engineered into the heterologous systems and blue light illumination was used to alter the cAMP-mediated cellular and behavioral responses of the systems [13; 14; 15]. PACs possess a BLUF (blue light using flavin) domain-coupled adenylate cyclase and is utilized for blue light-dependent modulation of cAMP [16; 17]. In 2007, Euglena PAC (ePAC) was expressed in a cell model system as well as in transgenic animals, and the cAMP level was manipulated simply by illumination with blue light [15; 18]. However, these large-sized (110 kDa) PACs showed significant cyclase activity in the dark. [PERSON] and coworkers characterized smaller-sized (38 kDa) PAC, which showed fast kinetics for light-regulated cyclase activity [13].