The Shewanella species are a diverse group of facultative anaerobic bacteria that are widely distributed in aquatic environments. The greatest asset of many Shewanella species is the ability to utilize a diverse set of electron acceptors that include toxic elements and insoluble metals [4]. The remarkably diverse respiratory versatility allows for survival in an array of environmental conditions. As such, these bacteria have been shown to facilitate bioremediation and in bioengineering applications [5,6]. In addition, Shewanella species typically form robust biofilm in diverse environments, such as on green algae [7] and mineral surfaces [8]. Biofilms are three dimensional bacterial communities that form on surfaces and provide protection from predation and environmental stressors, and, in the case of pathogenic bacteria, can be required for successful infection [9].

Bacterial biofilm formation is modulated by extracellular signals [10], including oxygen (O2). As Shewanella species can respire both aerobically and anaerobically, the bacteria must be able to change the terminal oxidant used in metabolism based on their environment [11]. In addition to Shewanella altering metabolism in response to O2 levels, many other bacteria have been shown to alter biofilm formation (surface attached bacterial communities), motility, and virulence, suggesting that O2 levels are monitored by many bacterial species [[12], [13], [14], [15], [16]].

A class of widely distributed heme proteins, termed globin coupled sensors, may be serving as environmental O2 sensors within bacteria and allowing for response to changing gas levels [15,[17], [18], [19]]. Globin coupled sensor (GCS) proteins consist of a N-terminal globin domain that is linked to a C-terminal output domain by a variable middle domain and activity of the output domain is modulated by diatomic gaseous ligands. GCSs contain many types of output domains [[20], [21], [22]], such as diguanylate cyclase domains [[15], [16], [17],23,24], which are responsible for synthesizing 3′,5′-cyclic dimeric guanosine monophosphate (c-di-GMP). C-di-GMP is a ubiquitous bacterial secondary messenger that plays a regulatory role in several cellular pathways and processes, such as biofilm formation [10,24]. Low levels of c-di-GMP have been shown to regulate the motile-sessile transition of bacteria and typically result in increased motility. Conversely, high levels of c-di-GMP production have shown to promote surface attachment and biofilm formation [25].

Diguanylate cyclase-containing GCS proteins have been identified across different bacterial genomes, suggesting widespread importance in regulating O2-dependent bacterial phenotypes. While the sensor globin and diguanylate cyclase domains of GCS proteins typically exhibit sequence homology, the middle domains of predicted GCS proteins vary in length (from ∼13–140 amino acids). To date, the majority of work on diguanylate cyclase (DGC)-containing GCS proteins has focused on those with long (∼130–140 amino acid) middle domains, such as the GCS proteins from Bordetella pertussis (BpeGReg; causative agent of whooping cough), Escherichia coli (EcDosC), and Pectobacterium carotovorum (PccGCS; soft rot plant pathogen) [17], while only one diguanylate cyclase-containing GCS with a shorter middle domain (∼40 amino acids) has been characterized (HemDGC from Desulfotalea psychrophila) [16]. Characterization of the ligand-dependent enzyme activity, conformation, and physiological role of a globin-coupled sensor from Shewanella sp. strain ANA-3 (SA3GCS), which is predicted to contain an N-terminal sensor globin connected by a ∼ 40 amino acid middle domain to a C-terminal diguanylate cyclase domain, has provided further insights into the role shortened middle domain on GCS signaling.