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Functional and Mechanistic Characterization of Bacterial Nitric Oxide Signaling Pathways

  • Author(s): Rao, Minxi
  • Advisor(s): Marletta, Michael A.
  • et al.
Abstract

Nitric oxide (NO) is a well-established signaling molecule and cytotoxic agent in mammals. NO is synthesized by nitric oxide synthase (NOS) by macrophages at high concentrations as a key part of the host immune response, and at low concentrations in endothelial and neuronal cells as a signaling agent. In endothelial cells, the primary NO receptor is soluble guanylate cyclase (sGC), which contains a heme-nitric oxide/oxygen binding domain (H-NOX). Selective binding of NO to the H-NOX domain is responsible for activation of sGC. Thus, the mammalian NO signaling system involves NO synthesis by NOS, and NO sensing by the H-NOX domain of sGC.

NOS and H-NOX proteins have also been identified in a number of bacterial species, including pathogens. Putative roles for bacterial NOS proteins include protection against oxidative stress and antibiotics, while bacterial H-NOX proteins have been shown to govern processes such as biofilm formation and bioluminescence via interactions with signaling proteins such as diguanylate cyclases (DGC) or histidine kinases (HK). Here, various aspects of NO signaling from three different organisms are characterized: the marine alphaproteobacterium Silicibacter sp. TrichCH4B; the soil-dwelling gammaproteobacterium Shewanella oneidensis, and the marine cyanobacterium Synechococcus sp. PCC 7335. This work and other recent studies seek to understand not only the diverse roles for NO in bacteria, but also the molecular mechanisms of bacterial NO signaling.

Silicibacter sp. TrichCH4B is the first bacterial organism discovered to contain both an NOS and H-NOX, thus capable of both NO synthesis and sensing, analogous to mammalian systems. The H-NOX protein from Silicibacter is found in an operon adjacent to an HK, forming part of a two-component phospho-relay signaling network. The response regulator of the network was identified to be a diguanylate cyclase (DGC), which is inactivated upon phosphorylation and establishes the link between NO and intracellular cyclic-di-GMP levels, and consequently biofilm formation. It was also determined that Silicibacter NOS activity is stimulated by a signaling protein from an algal symbiont, Trichodesmium erythraeum, which is a major marine nitrogen fixer. Thus, in the presence of Trichodesmium, the increase in NOS activity results in Silicibacter biofilm formation and poising the two species for nutrient exchange, revealing a novel role for NO in interspecies communication and symbiosis.

Given the diverse processes governed by NO/H-NOX signaling, it is crucial to understand the molecular mechanism by which H-NOX regulates HK autophosphorylation activity, the most common outcome of a NO-bound H-NOX. Here, the interaction and signal transduction between the H-NOX-HK signaling pair from Shewanella oneidensis are characterized. Binding kinetics measurements and analytical gel filtration revealed that NO-bound H-NOX has a tighter affinity for the HK, compared with H-NOX in the unliganded state, correlating binding affinity with kinase inhibition. Kinase activity assays with a panel of binding-deficient H-NOX mutants further reveal that while formation of the H-NOX-HK protein complex is required to stabilize the HK, H-NOX conformational changes upon NO binding are necessary for HK inhibition.

Characterization of H-NOX proteins has led to an increased understanding of bacterial NO sensing. However, NO production in bacteria is less well-understood, and here the NOS protein from Synechococcus sp. PCC 7335 is characterized. Mammalian NOS proteins are comprised of a P450-like heme/oxidase domain responsible for catalysis, and a reductase domain responsible for electron transfer. While most bacterial NOS proteins discovered to date contain only the heme/oxidase domain, Synechococcus NOS contains both the oxidase and reductase domains, and additionally contains a predicted globin domain resembling bacterial flavohemoglobins. Spectroscopic and biochemical characterization of the globin indicated a possible role in redox communication in this novel class of bacterial NOS enzymes.

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