Three adjacent operons, each concerned with photosynthesis in Rhodobacter capsulatus, have been shown by genetic means to be cotranscribable. In the course of describing the characteristics of the bchCA operon, which encodes two enzymes essential for bacteriochlorophyll synthesis, we found that the expression of the bchCA genes is influenced by readthrough from the upstream crtE and crtF genes. The crtE and crtF genes encode enzymes required for carotenoid biosynthesis and function as an operon. Furthermore, the distal structural gene of the bchCA operon, bchA, contains within it both the major oxygen-regulated promotor (Ppuf1) and the constitutive (Ppuf2) promotor for the puf operon. Since these three operons, crtEF, bchCA, and puf, are all transcribed in the same direction, it appears that polymerases traversing the downstream regions may start at any of several promoters. This pattern of transcription, which is unusual among bacteria, demonstrates that the activities of individual operons in a superoperonal cluster may be affected by their positions within the cluster.

A system for genetic exchange in Rhodopseudomonas capsulata has been discovered. Each genetic marker thus far examined can be transferred, and many strains of Rps. capsulata can participate in genetic exchange. The mechanism of gene transfer seems unlike that of any previously described bacterial system, since genes can be transferred by cell-free filtrates, but the vector is resistant to deoxyribonuclease and has a sedimentation constant of about 70 S.Nonsulfur purple photosynthetic bacteria manifest a remarkable range of metabolic plasticity, especially in regard to alternative modes of energy conversion. Certain species, particularly of the genus Rhodopseudomonas, grow rapidly, and in recent years such organisms have been increasingly used in studies on energy conservation processes, membrane formation, and metabolic regulation (1-3). Although biochemical mutants have been used in some of these researches, genetic approaches to elucidation of mechanisms have not been possible hitherto owing to our ignorance of genetic exchange processes in the nonsulfur purple bacteria. This report describes the discovery of a genetic recombination system in Rhodopseudomonas capsulata. The genetic transfer appears to be mediated by a novel vector. MATERIALS AND METHODSRhodopseudomonas capsulata, strain "St Louis," and the strains derived from it (Z-1, MI, and M5) have all been described (4); Rps. capsulata, strain KB1 was obtained from S. (n) Kaplan. Strains B6, B10, and H9 were isolated from pond and soil samples by standard enrichment techniques.The latter three isolates have been tentatively identified as Rps. capsulata on the basis of pigmentation, cell morphology, and especially the characteristic "zig-zag" arrangement of cells in chains.The medium used throughout was composed of 0.3% Bactopeptone and 0.3% Bacto yeast extract in deionized water; for preparation of solid media, either 0.6 or 1.2% Bacto-agar was used.Photosynthetic growth was achieved in screw-cap tubes filled to capacity; these were incubated at 350 and illuminated by a bank of three 60-watt lumiline bulbs (General Electric) at a distance of about three inches (7.62 cm). First, a rifampicin-resistant mutant and a streptomycinresistant mutant were selected from each strain. Media were inoculated with pairs of strains to be tested, one member of each pair bearing the streptomycin-resistance marker, the other, refampicin-resistance. After several generations of mixed growth, the frequency of simultaneous resistance to streptomycin and rifampicin was determined and compared to the same frequency measured in singly inoculated control cultures. This procedure revealed a pair of strains, H9 and B10, that consistently gave significantly more rifampicinand streptomycin-resistant colony-forming units (CFU) than would be expected from the frequency of doubly resistant colony-forming units in the control cultures (Table 1)

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