Macrocycle ring deformation as the secondary design principle for light-harvesting complexes

Natural light-harvesting is performed by pigment–protein complexes, which collect and funnel the solar energy at the start of photosynthesis. The identity and arrangement of pigments largely define the absorption spectrum of the antenna complex, which is further regulated by a palette of structural factors. Small alterations are induced by pigment–protein interactions. In light-harvesting systems 2 and 3 from Rhodoblastus acidophilus, the pigments are arranged identically, yet the former has an absorption peak at 850 nm that is blue-shifted to 820 nm in the latter. While the shift has previously been attributed to the removal of hydrogen bonds, which brings changes in the acetyl moiety of the bacteriochlorophyll, recent work has shown that other mechanisms are also present. Using computational and modeling tools on the corresponding crystal structures, we reach a different conclusion: The most critical factor for the shift is the curvature of the macrocycle ring. The bending of the planar part of the pigment is identified as the second-most important design principle for the function of pigment–protein complexes—a finding that can inspire the design of novel artificial systems.

Rhodoblastus sphagnicola sp. nov., a novel acidophilic purple non-sulfur bacterium from Sphagnum peat bog

An isolate of purple non-sulfur bacteria was obtained from an acidic Sphagnum peat bog and designated strain RST. The colour of cell suspensions of this bacterium growing in the light under anaerobic conditions is purplish red. Cells of strain RST are rod-shaped, 0.8–1.0 μm wide and 2.0–6.0 μm long, motile by means of polar flagella, reproduce by budding and have a tendency to form rosette-like clusters in older cultures. The cells contain lamellar intracytoplasmic membranes underlying, and parallel to, the cytoplasmic membrane. The photosynthetic pigments are bacteriochlorophyll a and carotenoids; the absorption spectrum of living cells shows maxima at 377, 463, 492, 527, 592, 806 and 867 nm. The cells grow photoheterotrophically under anaerobic or microaerobic conditions with various organic carbon sources or grow photolithoautotrophically with H2 and CO2. Strain RST is a moderately acidophilic organism exhibiting growth at pH values between 4.8 and 7.0 (with an optimum at pH 5.2–5.5). The major fatty acids are 16 : 1ω7c and 18 : 1ω7c; the major quinones are Q-10 and Q-9. The DNA G+C content of strain RST is 62.6 mol%. Analysis of the 16S rRNA gene sequence revealed that the novel isolate is most closely related (97.3 % sequence similarity) to the type strain ATCC 25092T of the moderately acidophilic purple non-sulfur bacterium Rhodoblastus acidophilus, formerly named Rhodopseudomonas acidophila. However, in contrast to Rbl. acidophilus, strain RST is not capable of aerobic growth in the dark, has no spirilloxanthin among the carotenoids and differs in the pattern of substrate utilization. The value for DNA–DNA hybridization between strain RST and Rbl. acidophilus ATCC 25092T is only 22 %. Thus, strain RST represents a novel species of the genus Rhodoblastus, for which the name Rhodoblastus sphagnicola sp. nov. is proposed. Strain RST (=DSM 16996T=VKM B-2361T) is the type strain.