Lowering of oxygen partial pressure in chemotrophic cultures or reduction of light intensity in phototrophic cultures of Rhodopseudomonas capsulata induced formation of the photosynthetic apparatus. A maximum of mRNA coding for the reaction center (RC) and the lightharvesting I B870 antenna complex polypeptides occurred 30 min after induction. Maximal expression of mRNA for B800-B850 antenna proteins appeared with a lag time of about 25 min after RC/B870 mRNA. Pigment-binding polypeptides were inserted into the membrane immediately after mRNA synthesis. It is concluded that the delayed formation of the B800-B850 complex compared to the RC and the B870 complex is caused by sequential expression of the corresponding genes. Biological activity of pigment-protein complexes increased after the incorporation of their polypeptides parallel to the maximum of bacteriochlorophyll synthesis. Studies on mutant strains defective in the formation of pigment-protein complexes suggested that pigment synthesis is of importance for assembly of stable complexes.Decreasing oxygen tension in chemotrophic cultures or decreasing light intensity in phototrophic cultures of facultative phototrophic bacteria induces formation of an intracytoplasmic membrane system and a preferential biosynthesis of the photochemical reaction center (RC) and light-harvesting complexes B870 and B800-B850 (reviewed in refs. 1-3). A stable assembly of these complexes takes place, provided that all polypeptides of the respective complexes and the pigments bacteriochlorophyll (BChl) and carotenoids are synthesized (4, 5).Recently it has been shown that decreasing oxygen tension in cultures of Rhodopseudomonas capsulata induces an increase of mRNAs specific for the RC subunits and B870 antenna polypeptides (6) and for the B800-B850 antenna polypeptides (7). These observations support the idea that the synthesis of the complex-forming polypeptides is under transcriptional control. The level of RNA for pigment synthesis shows only a small increase when oxygen tension is reduced (6,8).The genes for the M and L subunits of the RC and for the polypeptides of B870 are cotranscribed from the rxcA locus of the R. capsulata genome (9). The differential expression of RC and B870 genes has been explained by segmental differences in stability within the polycistronic rxcA transcript (9). Genes for the B800-B850 polypeptides lie outside this operon (10, 11). The structural genes for the B800-B850 a and 13 polypeptides have been cloned, and the nucleotide sequences have been determined (12). Polypeptides of the B800-B850 light-harvesting complex appear with a lag time after those of the RC and B870 complex in the membrane of R. capsulata (13).If the synthesis of complex-forming polypeptides is under transcriptional control, the genes for the B800-B850 polypeptides should be expressed sequentially after RC/B870 genes, and polypeptides should be detectable immediately after transcription of their genes. The results reported herein confirm these predictions. Experiments...

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The synthesis of the photosynthetic apparatus was induced in chemotrophically grown cultures of the wild type strain 37b4 of Rhodopseudomonas capsulata by lowering of oxygen partial pressure. In these induced but growth‐limited cultures the amount of total RNA per cell increased. Using specific DNA‐probes for genes of the light‐harvesting complex B800–850 and for ribosomal RNA it was shown that the levels of mRNA for the proteins of the light‐harvesting complex B800‐850 and for ribosomal RNA increased. In the mutant strain G1pho+, missing the B800–850 complex, but synthesizing small amounts of the M r 10000 polypeptide of the complex, the B800–850‐specific mRNA was not increased during induction. It is concluded that the synthesis of the B800–850 complex is under transcriptional control and that, under these conditions, the level of rRNA also increased.

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