INTRODUCTION
Vibrio cholerae, a Gram-negative bacterium, is the causative agent of cholera, leading to acute watery diarrhea (Clemens et al. , 2017). Outside of the host, V. cholerae resides in estuarine and coastal environments, and it has adopted multiple strategies to combat environmental challenges (Colwell, 1996). For example, V. cholerae has evolved robust response mechanisms to mitigate the potential adverse impacts of light. While photosynthetic organisms harness light for energy conversion, light can also cause photo-oxidative damage to living cells, negatively affecting nucleic acids, lipids, and proteins (Elias-Arnanz et al. , 2011). Given that UV radiation (100-380 nm wavelength) and blue light (380-470 nm) can effectively penetrate aquatic ecosystems (Häder et al. , 1998), V. cholerae , much like many of its marine counterparts, has developed mechanisms for sensing these wavelengths of light (Braatsch & Klug, 2004). This adaptation triggers the activation of a set of genes that encode proteins involved in photo-oxidative responses (Worthington et al. , 2003). Three such proteins have been characterized in the V. cholerae genome that belong to the cryptochrome/photolyase family (CPF) family: VCA0057 (Phr), a cyclobutane pyrimidine dimer (CPD) photolyase, VC1814 (Cry1), and VC1392 (Cry2), the latter two being ssDNA-specific photolyases (Selby & Sancar, 2006). Transcriptomic analysis revealed the profound impact of blue light exposure on V. cholerae as 6.3% of the microbe’s genes are differentially expressed in response to this stimulus. These three photolyase genes are among the highly induced genes (Tarduet al. , 2017).
The established transmission model for V. cholerae involves ingestion of contaminated water sources, followed by infection of the host, culminating in the return to aquatic reservoirs (Nelson et al. , 2009, Hsiao & Zhu, 2020). To establish colonization and cause disease, V. cholerae employs intricate signal transduction pathways to activate virulence factors (Matson et al. , 2007, Hsiao & Zhu, 2020). Alongside activating virulence genes, V. cholerae must also express factors necessary to survive toxic compounds produced by the host. Among these compounds, nitric oxide (NO) levels surge during infection, functioning as a free radical that disrupts proteins containing cysteine residues, iron-dependent enzymatic reactions, and components of the electron transport chain (Poole, 2005). In the host, NO is generated through acidified nitrite in the stomach and by enzymes like inducible nitric oxide synthase (iNOS) during inflammatory responses (Fang & Vazquez-Torres, 2019). Cholera patients exhibit heightened iNOS expression in their small intestines (Janoffet al. , 1997, Qadri et al. , 2002, Rabbani et al. , 2001, Chen, 2022), implying that V. cholerae encounters NO during human infection. To counteract the detrimental effects of toxic nitrogen species, V. cholerae employs a transcriptional regulator NorR to sense NO and activate hmpA , encoding a member of the flavohemoglobin family of enzymes that catalyzes the conversion of NO to nitrous oxide or nitrate (Stern et al. , 2012). NorR also activates the expression of nnrS, which encodes a novel protein that is important for RNS resistance (Stern et al. , 2013). Additionally, NO also impacts cellular signaling through S-nitrosylation of protein cysteine residues, including key virulence regulator AphB (Chen, 2022).
V. cholerae has evolved adaptive mechanisms to thrive across various challenging conditions during its transition from aquatic environments to the host gut and beyond. For instance, during initial infection, V. cholerae senses host signals and orchestrates both activation of virulence genes and repression of genes to evade host defenses (Hsiao et al. , 2006, Liu et al. , 2008, Yanget al. , 2013, Cakar et al. , 2018). Late in infection,V. cholerae activates a set of “late induced genes” to facilitate its dissemination into the aquatic environment (Kamp et al. , 2013, Schild et al. , 2007). However, the subsequent role of specific genes expressed during human infection in facilitating V. cholerae survival in its natural environment is less well known. In this study, we demonstrated that the photolyase Cry1 was upregulated in the presence of NO. We demonstrate that pre-induction of Cry1 by NO prior to exposure to blue light confers a survival advantage uponV. cholerae possessing functional Cry1. We speculate this NO-induced upregulation of Cry1 plays a pivotal role during human infection. In this scenario, V. cholerae encounters host-derived NO, and the subsequent induction of Cry1 effectively primes the bacterium for the aquatic environmental challenges, where blue light is abundant, once it exits the host gut.