Molecular-organization and biosynthesis of pigment-protein complexes of Rhodopseudomonas capsulata

Chlorophyll biosynthesis and assembly into chlorophyll-protein complexes in isolated developing chloroplasts

Isolated developing plastids from greening cucumber cotyledons or from photoperiodically grown pea seedlings incorporated (14)C-labeled 5-aminolevulinic acid (ALA) into chlorophyll (Chl). Incorporation was light dependent, enhanced by S-adenosylmethionine, and linear for 1 hr. The in vitro rate of Chl synthesis from ALA was comparable to the in vivo rate of Chl accumulation. Levulinic acid and dioxoheptanoic acid strongly inhibited Chl synthesis but not plastid protein synthesis. Neither chloramphenicol nor spectinomycin affected Chl synthesis, although protein synthesis was strongly inhibited. Components of thylakoid membranes from plastids incubated with [(14)C]ALA were resolved by electrophoresis and then subjected to autoradiography. This work showed that (i) newly synthesized Chl was assembled into Chl-protein complexes and (ii) the inhibition of protein synthesis during the incubation did not alter the labeling pattern. Thus, there was no observable short-term coregulation between Chl synthesis (from ALA) and the synthesis of membrane proteins in isolated plastids.

Formation of functional inter-species hybrid photosynthetic complexes in Rhodobacter capsulatus

A Rhodobacter capsulatus mutant strain deficient in all pigment-binding peptides and hence incapable of photosynthetic growth was genetically complemented with a plasmid-borne copy of the Rhodobacter sphaeroides puf operon. Hybrid reaction centers composed of R. sphaeroides L and M and R. capsulatus H subunits assembled in vivo, and host cells were photosynthetically competent. Light-harvesting complex B875, also encoded by the R. sphaeroides puf operon, was present along with the hybrid reaction center. These cells emitted fluorescence, however, indicating an impairment in energy transduction.

Control of photosynthetic membrane assembly in Rhodobacter sphaeroides mediated by puhA and flanking sequences

A reaction center H- strain (RCH-) of Rhodobacter sphaeroides, PUHA1, was made by in vitro deletion of an XhoI restriction endonuclease fragment from the puhA gene coupled with insertion of a kanamycin resistance gene cartridge. The resulting construct was delivered to R. sphaeroides wild-type 2.4.1, with the defective puhA gene replacing the wild-type copy by recombination, followed by selection for kanamycin resistance. When grown under conditions known to induce intracytoplasmic membrane development, PUHA1 synthesized a pigmented intracytoplasmic membrane. Spectral analysis of this membrane showed that it was deficient in B875 spectral complexes as well as functional reaction centers and that the level of B800-850 spectral complexes was greater than in the wild type. The RCH- strain was photosythetically incompetent, but photosynthetic growth was restored by complementation with a 1.45-kilobase (kb) BamHI restriction endonuclease fragment containing the puhA gene carried in trans on plasmid pRK404. B875 spectral complexes were not restored by complementation with the 1.45-kb BamHI restriction endonuclease fragment containing the puhA gene but were restored along with photosynthetic competence by complementation with DNA from a cosmid carrying the puhA gene, as well as a flanking DNA sequence. Interestingly, B875 spectral complexes, but not photosynthetic competence, were restored to PUHA1 by introduction in trans of a 13-kb BamHI restriction endonuclease fragment carrying genes encoding the puf operon region of the DNA. The effect of the puhA deletion was further investigated by an examination of the levels of specific mRNA species derived from the puf and puc operons, as well as by determinations of the relative abundances of polypeptides associated with various spectral complexes by immunological methods. The roles of puhA and other genetic components in photosynthetic gene expression and membrane assembly are discussed.

Molecular genetics of photosynthetic membrane biosynthesis in Rhodobacter sphaeroides

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Oxygen-regulated mRNAs for light-harvesting and reaction center complexes and for bacteriochlorophyll and carotenoid biosynthesis in Rhodobacter capsulatus during the shift from anaerobic to aerobic growth

The stability and regulation by oxygen of mRNAs for the photosynthetic apparatus in Rhodobacter capsulatus have been studied by using proflavin to inhibit transcription and by shifting cells from anaerobic to aerobic conditions. The results from the inhibition experiments show that the mRNA for the light-harvesting LH-II polypeptides (beta, alpha) is more stable than that for the light-harvesting LH-I polypeptides (beta, alpha) during anaerobic growth, whereas the mRNAs for the reaction center polypeptides L (RC-L), M (RC-M), and H (RC-H) are less stable than both the LH-I and LH-II mRNAs. When photosynthetic cells are shifted from anaerobic to aerobic conditions, an immediate decrease in the levels of mRNA for the LH-I, LH-II, RC-L, RC-M, and RC-H proteins was observed. The level of mRNA for the LH-II proteins, however, is more sensitive to oxygen and is reduced faster than the level of mRNA for the LH-I proteins. These results suggest that oxygen represses the expression of genes coding for the light-harvesting antenna and reaction center complexes and may selectively accelerate the degradation of mRNA for the LH-II proteins. The mRNAs for several enzymes in the bacteriochlorophyll biosynthetic pathway are regulated by oxygen in a similar manner. The mRNAs for carotenoid biosynthetic enzymes, however, are regulated by oxygen in a different way. We have found that the amounts of mRNAs for carotenoid biosynthetic enzyme, relative to the amounts of mRNAs for LH and RC, increased during the shift from anaerobic to aerobic conditions. We have particularly shown that although the expression of most photosynthetic genes in R. capsulatus is repressed by oxygen, the crtA gene, located in the BamHI H fragment of the R' plasmid pRPS404 and responsible for the oxidation of spheroidene to spheroidenone, responds to oxygen in an opposite fashion. This exzymatic oxidation may protect the photosynthetic apparatus from photooxidative damage.

Structure and functional organization of light-harvesting complexes and photochemical reaction centers in membranes of phototrophic bacteria

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