Prokaryote Transmembrane Signaling

One-component systems dominate signal transduction in prokaryotes 
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"By analyzing information encoded by 145 prokaryotic genomes, we found that the majority of signal transduction systems consist of a single protein that contains input and output domains but lacks phosphotransfer domains typical of two-component systems. One-component systems are evolutionarily older, more widely distributed among bacteria and archaea, and display a greater diversity of domains than two-component systems.

Signal transduction pathways in prokaryotes regulate cellular functions in response to environmental cues. According to the current view, prokaryotic signal transduction is conducted mostly by two-component regulatory systems that function as a result of phosphotransfer between two key proteins: a sensor histidine kinase and a response regulator [1-6]. Most histidine kinases that have been experimentally studied are membrane-bound and have an extracellular input domain, whereas all response regulators are cytosolic. In a typical two-component system, the input domain of the sensor histidine kinase detects the environmental signal (usually a small molecule ligand) resulting in the activation of the histidine kinase domain, which autophosphorylates at a specific histidine residue (H; Figure 1a). The phosphoryl group (P) is then transferred to a specific aspartate residue (D) in the receiver domain of a response regulator. Phosphorylation of the response regulator activates the output domain, which initiates the corresponding cellular response. The majority of experimentally characterized two-component systems regulate gene expression at the level of transcription using the DNA-binding helix-turn-helix (HTH) output domains of the response regulators [4,5]. In addition, some response regulators contain enzymatic output domains, such as the di-guanylate cyclase [7].

... it is possible that the last common ancestor of archaea and bacteria (i.e. the last common ancestor of all modern life forms) did not have two-component systems, but encoded several one-component regulators.

The availability of a large number of sequenced prokaryotic genomes has enabled the dominance of one-component signal transduction systems to be revealed and the apparent ancestor-descendant relationship between them and the two-component systems in prokaryotes to be proposed.

Evolution and phyletic distribution of two-component ... 
Only abstract available online.  I got the PDF through the library. 
"Two-component signal transduction systems are abundant in prokaryotes. ... These systems are also found, although in much smaller numbers, in lower eukaryotes and plants..."  "...absence of the systems in mammals..."  "As their name suggests, two-component signal transduction systems are composed of two dedicated proteins, a sensor and a regulator [14..]"  "The majority of sensing and signal transduction in prokaryotes appears to be carried out by so-called one-component systems (OCSs), single proteins that combine properties of both a sensor and a regulator [10..].

Evolutionary origin of cholinergic macromolecules ... (PubMed) 
Full length article online. 
Our results demonstrate that the record of evolutionary history for nervous system proteins can be read across the boundaries of separation between vertebrates and invertebrates. They also provide molecular evidence for the common evolutionary origins of the nervous and endocrine systems in vertebrates--both evolving to make intercellular communication possible.

I'm not sure, but I think the "
common evolutionary origins", referred to above, are the "common evolutionary origins" of hormones, neuromodulators and neurotransmitters.