Peripheral Light-Harvesting LH2 Complex Can Be Assembled in Cells of Nonsulfur Purple Bacterium Rhodoblastus acidophilus without Carotenoids.

The effect of carotenoids on the assembly of LH2 complex in cells of the purple nonsulfur bacterium Rhodoblastus acidophilus was investigated. For this purpose, the bacterial culture was cultivated with an inhibitor of carotenoid biosynthesis - 71 µM diphenylamine (DPA). The inhibitor decreased the level of biosynthesis of the colored carotenoids in membranes by ~58%. It was found that a large amount of phytoene was accumulated in them. This carotenoid precursor was bound nonspecifically to LH2 complex and did not stabilize its structure. Thermostability testing of the isolated LH2 complex together with analysis of carotenoid composition revealed that the population of this complex was heterogeneous with respect to carotenoid composition. One fraction of the LH2 complex with carotenoid content around 90% remains stable and was not destroyed under heating for 15 min at 50°C. The other fraction of LH2 complex containing on average less than one molecule of carotenoid per complex was destroyed under heating, forming a zone of free pigments (and polypeptides). The data suggest that a certain part of the LH2 complexes is assembled without carotenoids in cells of the nonsulfur bacterium Rbl. acidophilus grown with DPA. These data contradict the fact that the LH2 complex from nonsulfur bacteria cannot be assembled without carotenoids, but on the other hand, they are in good agreement with the results demonstrated in our earlier studies of the sulfur bacteria Allochromatium minutissimum and Ectothiorhodospira haloalkaliphila. Carotenoidless LH2 complex was obtained from these bacteria with the use of DPA (Moskalenko, A. A., and Makhneva, Z. K. (2012) J. Photochem. Photobiol., 108, 1-7; Ashikhmin, A., et al. (2014) Photosynth. Res., 119, 291-303).

Isolation of novel bacteria and actinomycetes using soil-extract agar medium.

Novel bacteria were discovered using an isolation technique consisting of (i) selection of microorganisms that grew on soil-extract agar medium, but not on conventional media, and (ii) detection of small microbial colonies with a microscope. Three bacterial strains thus isolated were provisionally designated Shinshu-th1, -th2, -th 3, and five actinomycete strains, Shinshu-MS-01, -02, -03, -04, -05, respectively. Sequence analysis of their 16S rDNA showed that th1 had 95--96% homology with three unculturable bacteria, and th2 had 96% similarity to Bradyrhizobium sp., one unculturable and one unidentified bacterial strain. A phylogenetic study indicated that both strains were alpha-Proteobacteria belonging to the order Rhizobiales and the family Bradyrhizobiaceae. Since they had low homology (96%) with their close relatives, it is possible that th1 and th2 belong to a new genus. The actinomycetes Shinshu-MS-02 and -03 had 95--96% homology with four strains of Actinomadura, -04 had 95--96% similarity to Streptosporangium and Microbispora, and -05 had 97--98% homology with three strains of Acrocarpospora, Herbidospora and Planotetraspora. According to the phylogenetic study, both 02 and 03 are possibly new species of Actinomadura, -04 of Streptosporangium, and -05 of Acrocarpospora. Shinshu-th 3 and -MS-01 were identified as Mycobacterium cookii and Frankia sp., respectively, having 99% homology with these species.