Photosensitivity of Chromatophores

Almost all major phyla of invertebrates and lower vertebratesdisplay a direct sensitivity of their chromatophores to lightby either dispersing or—in rare cases—aggregatingthe pigment granules within the cell. This "primary response"of color change is an accessory component of the "dermal lightsense" and characterizes the chromatophores as an independentreceptor for light and effector of the chromomotor response.Photosensitivity does not seem to be restricted to certain partsof the pigment cell but is rather supposed to be a ubiquitousproperty of the chromatophore. Experiments on partially illuminatedchromatophores show that the photopigment, whose chemical compositionis still unknown, is localised within the plasma membrane orthe cytoplasmic ground substance. Threshold responses for ajust visible reaction are much higher than for background responsesand have for some pigment cells been determined to be in a rangebetween 0.2 and 0.5 erg/sec/cm². The physiological significanceof the photosensory reaction is closely related with thermoregulationand the protection of underlying tissue against harmful radiation.The chain of events involved in photosensory transduction remainsto be further studied and can at present be interpreted onlyon the basis of related phenomena like retinal pigment migrationand the light-sensitivity of simple non-pigmented contractilesystems.     

Fluorescence induction of photosynthetic bacteria

The kinetics of bacteriochlorophyll fluorescence in intact cells of the purple nonsulfur bacterium Rhodobacter sphaeroides were measured under continuous and pulsed actinic laser diode (808 nm wavelength and maximum 2 W light power) illumination on the micro- and millisecond timescale. The fluorescence induction curve was interpreted in terms of a combination of photochemical and triplet fluorescence quenchers and was demonstrated to be a reflection of redox changes and electron carrier dynamics. By adjustment of the conditions of single and multiple turnovers of the reaction center, we obtained 11 ms^–1 and 120 μs^–1 for the rate constants of cytochrome c₂³⁺ detachment and cyclic electron flow, respectively. The effects of cytochrome c₂ deletion and chemical treatments of the bacteria and the advantages of the fluorescence induction study on the operation of the electron transport chain in vivo were discussed.     

Bacteriophytochromes in anoxygenic photosynthetic bacteria

Since the first discovery of a bacteriophytochrome in Rhodospirillum centenum, numerous bacteriophytochromes have been identified and characterized in other anoxygenic photosynthetic bacteria. This review is focused on the biochemical and biophysical properties of bacteriophytochromes with a special emphasis on their roles in the synthesis of the photosynthetic apparatus.     

Nitrogen fixation by photosynthetic bacteria

In 1949, Howard Gest and Martin Kamen published two brief papers in Science that changed our perceptions about the metabolic capabilities of photosynthetic bacteria. Their discovery of photoproduction of hydrogen and the ability of Rhodospirillum rubrum to fix nitrogen led to a greater understanding of both processes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evolutionary relationships among photosynthetic bacteria

To understand the evolution of photosynthetic bacteria it is necessary to understand how the main groups within Bacteria have evolved from a common ancestor, a critical issue that has not been resolved in the past. Recent analysis of shared conserved inserts or deletions (indels) in protein sequences has provided a powerful means to resolve this long-standing problem in microbiology. Based on a set of 25 indels in highly conserved and widely distributed proteins, all main groups within bacteria can now be defined in clear molecular terms and their relative branching orders logically deduced. For the 82 presently completed bacterial genomes, the presence or absence of these signatures in various proteins was found to be almost exactly as predicted by the indel model, with only 11 exceptions observed in 1842 observations. The branching order of different bacterial groups as deduced using this approach is as follows: low G+C Gram-positive (Heliobacterium chlorum) ↔ high G+C Gram-positive ↔ Clostridium–Fusobacterium–Thermotoga ↔ Deinococcus–Thermus ↔ green nonsulfur bacteria (Chloroflexus aurantiacus) ↔ Cyanobacteria ↔ Spirochetes ↔ Chlamydia–Cytophaga–Flavobacteria–green sulfur bacteria (Chlorobium tepidum) ↔ Aquifex ↔ Proteobacteria (δ and ∈) ↔ Proteobacteria (α) ↔ Proteobacteria (β) and ↔ Proteobacteria (γ). The Heliobacterium species, which contain an Fe–S type of reaction center (RC 1) and represent the sole photosynthetic phylum from the Gram-positive or monoderm bacteria (i.e., bounded by only a single membrane), is indicated to be the most ancestral of the photosynthetic lineages. Among the Gram-negative or diderm bacteria (containing both inner and outer cell membranes) the green nonsulfur bacteria, which contain a pheophytin-quinone type of reaction center (RC 2), are indicated to have evolved first. The later emerging photosynthetic groups which contain either one or both of these reaction centers could have acquired such genes from the earlier branching lineages by either direct descent or by means of lateral gene transfer. This revised version was published online in August 2006 with corrections to the Cover Date.