During the past three years we have studied in depth a photosynthetic gene cluster from the non-sulfur purple bacterium Rhodopseudomonas capsulata. The structural genes for the two light harvesting I polypeptides have been sequenced. The genes for the three reaction center polypeptide subunits, H, M, and L, have been sequenced. In addition, thirty Tn5.7 transposon mutants in the photosynthetic gene cluster have been isolated, mapped, and phenotypically characterized. We have also observed an remarkable sequence homology between the M and L subunits of Rps. capsulata and the D2 and D1 polypeptides which are coded on the chloroplast genomes of higher plants and algae. Based on this homology and chemical intuition, we have formulated a model for the quinone binding sites of photosynthetic reaction centers. We propose the continuation of each of these three areas of major development. Using reverse transcription on isolated messenger RNA templates from synthesized oligonucleotide primers, the sequenced areas of the photosynthetic gene cluster will be mapped with respect to their operon structures and the positions and sequences of their promoter sites. The distinctions between O2 regulated promoter and constitutive promoter sequences will be established and ultimately the basis for this distinction at the level of RNA polymerase will be determined. Starting with the gene map for the bacteriochlorophyll and catotenoid biosynthetic pathway provided by the transposon mutagenesis, we intend to associate a defined enzymatic activity with each gene locus. The operon map for these unsequenced regions of the photosynthetic gene cluster will also be determined. Finally, the quinone binding regions of the photosynthetic membrane will be determined by affinity labelling techniques utilizing an newly developed class of suicide quinones that are activated only by the natural quinone reductases which exist in the photosynthetic membrane.