Sensory Physiology

Sensory Physiology

Q.1

Ans

Eye

All photosynthetic organisms possess finely tuned capacities to sense and respond to changes in their environment. The perception of light and the developmental changes that occur as a response to light signals are arguably the most important adaptive responses of any organism that uses light for carbon fixation and generation of reductant. Thus, photosynthetic organisms contain a range of proteins that allow them to detect and respond to light (Alvey, Li, Balabas, Seib, Stowe-Evans, & Kehoe, 2005), including photosynthetic and photosensory proteins.

As the sequencing of prokaryotic genomes has progressed, it has become apparent that many bacterial systems possess photoreceptors that are related in sequence and function to eukaryotic photoreceptors. As these data began to emerge, it had already been noted that complementary chromatic adaptation, a specific form of photomorphogenesis in some cyanobacteria, was under the control of a biliprotein photoreceptor with significant similarity to plant phytochromes (Kehoe and Grossman, 1996). In higher plants, the biliprotein phytochromes covalently bind a linear tetrapyrrole chromophore (bilin) and control many aspects of growth and development, from seed germination through senescence. Although phytochromes and phytochrome-related proteins have been found in a wide range of organisms from eubacteria through higher plants, cyanobacteria contain a larger number of proteins harbouring chromophore-binding GAF domains than any other group of organisms examined to date (Ohmori et al., 2001; Okamoto and Ohmori, 2003; see description of GAF domains in Box 1). These organisms also contain three recognized classes of flavin-based blue light receptors: cryptochromes, phototropin-like photoreceptors and BLUF proteins (van der Horst and Hellingwerf, 2004). Cyanobacterial genomes also possess genes encoding sensory rhodopsins (reviewed by Spudich, 2006) and other novel photosensory proteins. In recent years, a great deal of progress has been made in the study of cyanobacterial photoreceptors and light sensing in these organisms.

Overview of Photosensing and Photomorphogenesis in Cyanobacteria

Cyanobacteria are aquatic, Gram-negative prokaryotes that range from unicellular/colonial to filamentous to branched filamentous in form. Cyanobacteria are autotrophic and utilize oxygen-evolving photosynthesis to produce fixed carbon, as do higher plants. Cyanobacteria possess photosynthetic light-harvesting antennae called phycobilisomes. These phycobilisomes contain light-absorbing phycobiliproteins that have covalently attached, linear tetrapyrrole chromophores. The phycobiliproteins absorb light and transfer light energy to photosystem II for photosynthesis.

In addition to carrying out photosynthesis, cyanobacteria exhibit many acclimation or adaptation responses to light. For example, many cyanobacteria possess the ability to alter dramatically the composition of their phycobilisomes in response to changes in the prevalent wavelengths of light in their ambient environment (Palenik, 2001; Everroad et al., 2006; Kehoe and Gutu, 2006). This ability, termed chromatic adaptation, is exhibited in a variety of forms in different species of cyanobacteria. Cyanobacteria also move in response to light in a process called phototaxis.

Cyanobacterial sensory proteins initiate a signal transduction cascade in response to an environmental signal. These signalling cascades generally consist of three steps: signal perception, signal transduction and cellular responses (see Fig. 1). Many of these pathways are phosphotransfer cascades commonly known as two-component systems. Two-component systems are based on two signal transduction components, including ...