Chapter 2 - The Simplest Eyes: Rhodopsin-mediated Phototaxis Reception in Microorganisms

Abstract

The conservation of residues in microbial rhodopsins, especially in the retinal-binding pocket, defines a large phylogenetic class called type 1 rhodopsins to distinguish them from the visual pigments and related retinylidene proteins in higher organisms (type 2 rhodopsins). The sequence of the newly found type 1 rhodopsins, their heterologous expression and study, and, in some cases, the study of the photosensory physiology of the organisms containing them have shown that the newfound pigments fulfill both ion transport and sensory functions, the latter with a variety of signal-transduction mechanisms. Microbial rhodopsins functioning as light-driven proton pumps are widespread in prokaryotic and eukaryotic species. Phylogenetic analysis strongly suggests that microbial rhodopsin photosensors evolved from the proton pumps and that this evolutionary event occurred multiple times in different lineages independently. The reasoning is that the pumps, which have a single protein function without a need for interaction with other proteins, readily undergo lateral gene transfer, followed by duplication and modification to develop a functional interaction with signal-transduction machinery of the new host. Such parallel evolution of sensory rhodopsins fits the experimental observation that their signaling mechanisms are strikingly different in different branches of the phylogenetic tree.