Diversity of bacterial chemosensory systems

Abstract

Chemosensory system is the most complex, specialized mode of signal transduction in bacteria and archaea. It is composed of several core and auxiliary protein components that are highly organized in order to deliver a fast response to changing environmental conditions. Chemosensory pathways were studied in-depth in a handful of model organisms and experimentally characterized at least to some degree in approximately thirty other species. However, genome-wide analyses have revealed their presence in thousands of sequenced microbial genomes. Both experimental and computational studies uncovered substantial diversity in system design, functional regulation, cellular localization and phyletic distribution of chemosensory pathways. Here, we summarize advances and expose gaps in our current understanding of the diversity of chemosensory systems.

Figure 1.. Chemosensory systems as complex type of two-component signal transduction.

Prototypical two-component systems consists of two proteins: a transmembrane sensor histidine kinase, which contains an input (e.g. ligand-binding) domain and a cognate response regulator, which contains an output (e.g. DNA-binding) domain. A chemosensory system of E. coli has the sensor-less (no input domain) histidine kinase CheA and its response regulator CheY, which lacks an output domain and directly interacts with the flagellar motor. Abbreviations: M, MCP; W, CheW; A, CheA; B, CheB; R, CheR; Y, CheY; Z, CheZ; P, a phosphate group.

Figure 2.. Major classes of the chemosensory system.

Model organisms in which corresponding chemosensory pathways were studied experimentally are shown for each class. Typical architectures are displayed, but protein repertoire within each class may vary. Tfp, ACF and F9 pathways typically contain a single MCP, whereas multiple MCPs are associated with all other classes. Core essential components (M, MCP; A, histidine kinase CheA; W, scaffolding protein CheW; Y, response regulator CheY) are in light grey; core dispensable components (R, methyltransferase CheR; B, methylesterase CheB) are in dark grey; the auxiliary proteins (D, deamidase CheD; V, scaffolding protein CheV; C, Z and X, phosphatases CheC, CheZ and CheX, respectively) and output domain of response regulator protein from ACF system are shown in color. Experimentally confirmed protein-protein interactions are represented by solid lines; dashed lines indicate predicted interactions.

Figure 3.. Phyletic distribution of experimentally studied bacterial chemosensory systems.

The genome tree showing all bacterial phyla with at least 10 sequenced genomes was adapted from the Genome Taxonomy Database [49]. Organisms with experimentally studied chemosensory systems are mapped to their respective phyla. All known types of chemosensory systems are listed for each organism.