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The recently discovered gene transfer system of Rhodopseudomonas capsulata was used to construct a genetic map of a region concerned with bacteriochlorophyll and carotenoid production. Mutants blocked in the biosynthesis of these compounds were isolated, and each was characterized on the basis of pigments accumulated during growth under low PO2. One-point, two-point, three-point, and ratio test crosses were performed between various mutant strains, and the results were amenable to conventional genetic analyses. A mapping function was found that related cotransfer frequency to map distance. Seven clusters of mutations, five affecting carotenoid and two affecting bacteriochlorophyll biosynthesis, were arranged in one linkage group. Each cluster of mutations is thought to represent a gene. The length of the mapped region is estimated to be less than 1% of the genome. Cotransfer is observed between markers separated by about 5 to 10 genes.

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Many strains of Rhodopseudomonas capsulata are capable of exchanging genetic information via a recently discovered gene transfer process involving the release and subsequent uptake from the medium of particles containing genetic information (gene transfer agents, GTAs). No viral activities are observed to be associated with this system. An assay has been developed to quantitate gene transfer in R. capsulata. Conditions are described for which the number of cells acquiring a new genetic trait is directly proportional to the number of GTAs and independent of the number of recipient cells. These conditions were used for the assay of the uptake and release of GTAs by cells. The maximum fraction of recipients that acquire a given genetic marker is -4 x 10-4. Free GTA appears in a growing culture in one or two abrupt waves near the time of transition from exponential to stationary phase. During these waves, the titer of GTA for a given marker may reach 2 x 106/ml. A comparison of the frequency of singleand double-marker transfers suggests that most of the cells in early-stationaryphase cultures are active recipients. The ultraviolet inactivation spectrum of GTA resembles that of the small ribonucleic acid phages. The inactivation cross section a, for GTA was calculated to be 1.7 x 10-1 cm 2/photon at 265 nm.We have recently described the discovery of the first genetic exchange system for a nonsulfur purple photosynthetic bacterium (3). Certain strains of Rhodopseudomonas capsulata are capable of releasing particles that contain samples of genetic information representing all parts of the genome. These particles can subsequently provide genetic information to other bacteria of the same species, thus effecting genetic transfers. The process resembles transduction; however, the particles are much smaller than any known transducing bacteriophage. We propose to call these particles gene transfer agents (GTAs), and cells that express new genetic markers received via GTAs are termed transferants. It is our goal to be able to use this genetic system to study energy metabolism and the regulation of photosynthetic membrane formation via a biochemical genetics approach. This report describes the development of a quantitative assay for R. capsulata gene transfer, the use of the assay to study some of the characteristics of the system, and further evidence about the physicochemical nature of the vector mediating the gene transfer.MATERIALS AND METHODS Bacterial strains. R. capsulata B10 and B6 have been described previously (3). The following strains were derived from B10: BB101 (carries marker rif-10 which confers resistance to rifampin [RifR]); BB102 (carries marker str-1 which confers resistance to streptomycin [StrRI); BB1012 (carries rif-10 and str-1); YB1020 (carries str-2 which confers streptomycin resistance and trpA20 which causes tryptophan auxotrophy [Trp ]).Media. Peptone-yeast extract medium (PYE; 0.3% peptone [Difco] and 0.3% yeast extract [Difco] in deionized water) was used for growth of recipient and donor cultures...

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