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.