Introduction

Bacteria and Archaea have survived the most dramatic environmental changes that have occurred since their first appearance, three billion years ago. They have occupied almost every ecological niche available, including extremes such as high temperatures at acidic or alkaline conditions. One reason for their endurance is their ability to respond adequately and precisely to environmental changes either genetically or by a locomotive answer. The information flow from the external input across the plasma membrane to the activation of the physiological signal is based on the so-called two-component signalling system that has been found in all three domains of life (for recent reviews on eukaryotic and prokaryotic two-component system see, e.g., [1-6]). This signalling pathway consists of sensors, which receive and transmit the external stimuli to cytoplasmic proteins, including both a histidine and an aspartate kinase (hence the name) which function as transmitter and receiver, the latter regulating the physiological response on the level of genes, proteins, or the cellular motor. The input signal can be quite diverse, ranging from magnetic fields, gravity, or osmolarity to chemicals, starvation, or photons, to name a few.

The two-component signalling cascade has been thoroughly investigated for the chemosensory system of Escherichia coli, Salmonella typhimurium, and related enteric bacteria. In recent years a similar signalling cascade from the archaeal Halobacterium salinarum has been discovered while analysing the mechanism of phototaxis. These archaea have been of particular interest since the discovery of bacteriorhodopsin (BR), the light-activated proton pump, in the early 1970s [7]. The wealth of available information on the function and structure of BR has been reviewed (e.g. [8]; see also a special issue of Biochem. Biophys. Acta, 1460 (2000) with a comprehensive discussion of BR and related pigments). Various three-dimensional structures of the BR ground state [9-11] and intermediates [11-13] (reviewed in [14]) are now accessible and provide a basis for the understanding of the molecular mechanism of the light-activated proton transfer. Furthermore, this data is important in elucidating signal transduction as exemplified in the sensory rhodopsins.

During these investigations on BR three other retinylidene proteins were discovered. Halorhodopsin (HR), an ion pump like BR (both reviewed e.g. in [15] and [16]), was first described and named by Mukohata and co-workers [17]. In subsequent work, HR has been recognised as an inward directed chloride pump [18] and the amino acid sequence has been determined [19]. Since 2000 the tertiary structure of HR has been available at 1.8 A resolution [20], The other two pigments, sensory rhodopsin I (SRI) and sensory rhodopsin II (HsSRII), are responsible for the phototaxis of the bacteria and enable them to seek optimal light conditions for the functioning of the ion pumps HR and BR (SRI) and to avoid photo-oxidative stress (HsSRII) [21] (Figure 1). The earliest

Sensory Rhodopsins Ion pumps

Figure 1. The four archaeal rhodopsins as molecular models. The structures depicted were taken from Sass et al. (BR) [13], Luecke et al. (for SRI and HsSRII the structure of NpSRII was taken [124]) and Kolbe et al. (HR) [20], The receptors SRI and HsSRII are bound to their cognate transducers, forming a 2:2 complex. For the dimeric structure of the transducer the model of the serine chemotaxis receptor was taken [87], The models are not drawn to scale. Approximate distances are indicated. In the lower panel an electron microscopic picture of H. salinarum is depicted, showing the bacterium with its polarly inserted flagella. [Electron micrograph adapted from [42]].

Figure 1. The four archaeal rhodopsins as molecular models. The structures depicted were taken from Sass et al. (BR) [13], Luecke et al. (for SRI and HsSRII the structure of NpSRII was taken [124]) and Kolbe et al. (HR) [20], The receptors SRI and HsSRII are bound to their cognate transducers, forming a 2:2 complex. For the dimeric structure of the transducer the model of the serine chemotaxis receptor was taken [87], The models are not drawn to scale. Approximate distances are indicated. In the lower panel an electron microscopic picture of H. salinarum is depicted, showing the bacterium with its polarly inserted flagella. [Electron micrograph adapted from [42]].

report on the phototactic behaviour of H. salinarum was published in 1975 [22] although the involvement of retinal proteins was only recognised in subsequent work [23-26]. At about the same time it was demonstrated that methylation of membrane proteins is involved in the photosensory and chemosensory behaviour of H. salinarum [27-29] which suggested that a sensory pathway similar to that in E. coli exists.

Research into halobacterial photosensing made a decisive step forward when Spudich and Spudich isolated HR-deficient mutants [30]. These so-called flux mutants were obtained by exciting HR in cells in which a small proton leak had been introduced with a protonophore. The method selects for mutants which escape cytoplasmic acidification. In such a way isolated mutants lacking BR as well as HR were used for phototaxis studies. The photo-sensory behaviour of these bacteria was unimpaired, demonstrating that neither BR nor HR are involved in phototaxis [31] (however, see below for more recent experiments on BR as photosensor) [32]. The authors identified a retinal-containing protein absorbing between 580 and 590 nm. It was named 'slow rhodopsin-like pigment' (later renamed as sensory rhodopsin I; SRI) because of its photocycle turnover of 800 ms, in contrast to that of about 10 ms for BR or HR.

On light excitation SR forms, in analogy to the BR-photocycle, a long-lived intermediate with a fine-structured absorption band with a maximum at 373 nm. This species is also photoactive and has been correlated with the photo-phobic response of H. salinarum. The notion that the same photoreceptor is responsible for both the repellent as well as the attractant responses has been further elaborated by the same authors [33]. The observations were summarised in a mechanism of colour sensing mediated by a single receptor (SRI). The essence of the model is the discrimination between visible and UV light by one- and two-photon processes, respectively. The absorption of a photon (A > 500 nm) by SRI triggers the photocycle, which results in the activation of the attractant signal transduction chain. However, in the presence of both visible and UV light the long-lived intermediate (S373) is excited and the repellent signalling cascade is turned on. This proposal of Spudich and Bogomolni was confirmed in later work and is now the accepted explanation for the colour discrimination of H. salinarum.

During further work on the halobacterial phototaxis, another repellent pigment was identified, named phoborhodopsin (pR) [34] or sensory rhodopsin II (HsSRII) [35]. HsSRII covers the blue-green region of the spectrum. Its photocycle, like that of SRI, is quite slow and also contains, similar to BR, an M-like intermediate. Contrary to SRI, this pigment induces in H. salinarum only a single answer to light, i.e. a photophobic response. The four archaeal rhodopsins detected in H. salinarum are depicted in Figure 1. The corresponding amino acid sequences are shown in Figure 2.

The amino acid sequences of the two sensory rhodopsins have been determined [36,37]. Additionally, the primary structures of HsSRII from the archaeal species Natronobacterium pharaonis (NpSRII) and Haloarcula vallismortis are available [38], The amino acid sequences of the SRs reveal

Cytoplasm

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