Evolution and phyletic distribution of two-component signal transduction systems

Abstract

Two-component signal transduction systems are abundant in prokaryotes. They enable cells to adjust multiple cellular functions in response to changing environmental conditions. These systems are also found, although in much smaller numbers, in lower eukaryotes and plants, where they appear to control a few very specific functions. Two-component systems have evolved in Bacteria from much simpler one-component systems bringing about the benefit of extracellular versus intracellular sensing. We review reports establishing the origins of two-component systems and documenting their occurrence in major lineages of Life.

Figure 1

Prokaryotic signal transduction paradigms. Extracellular sensing emerged with the advent of TCSs given that 73% of HKs are membrane associated compared to only 3% of OCSs [10•]. However, both OCSs and TCSs are predicted to predominantly regulate transcription [10•]. The chemotaxis system (represented by the canonical E. coli system) employs a number of additional components not reported in other TCSs that regulate methylation (M) (CheB and CheR), dephosphorylation (CheZ), and scaffolding (CheW).

Figure 2

Modularity of TCS domain architectures. A survey of TCS component domain architectures (mistdb.com) reveals extensive diversity, some of which is represented here. Receiver domains can be found tandemly repeated in many of these architectures. Similarly, there can be multiple sensor domains in RRs and HRRs in addition to HKs and HHKs. These components can be combined in different ways to form a variety of phosphorelay pathways [2•,6], which supports the importance of distinguishing between HHKs and HRRs when considering such possibilities [4•].