The thermotolerant nitrogen-fixing bacterium Rhodospirillum centenum, isolated in 1987, is unique among photosynthetic prokaryotes in that it produces desiccation- and heat- resistant cysts. R. centenum is the only Rhodospirillum species known to be amenable to facile genetic and molecular biological analysis. More than 100 mutants with defects in photosynthetic metabolism have been isolated, and these will be characterized biochemically as part of a major objective, namely, mapping the order of the photosynthesis genes of this organism. Determination of the gene order will provide data of importance for future work on "photochemical reaction centers" and for assessing the possible significance of "lateral gene transfer" in the evolution of photosynthetic systems. Control of expression of photosynthesis genes in R. centenum will also be investigated, especially in regard to effects of molecular oxygen. Because the availability of an increased range of unusual phenotypes provides experimental systems of value for study of a number of fundamental problems, searches for previously unknown biotypes of non- oxygenic photosynthetic bacteria will continue using various selective enrichment and other procedures. Anoxygenic photosynthetic bacteria are widely distributed in nature and have been isolated from a great variety of ecological niches. In certain locales they have been identified as primary producers of organic matter and they are prominent catalysts in various geochemical cycles. They have also emerged as outstanding experimental systems for analysis of the basic mechanisms of energy conversion in all types of photosynthetic organisms. It is important to decipher the evolutionary relationships between these organisms. Using both classical genetics and molecular biology, these relationships will be explored with the recently discovered bacterium, Rhodospirillum centenum.
