Integration of Energy Conversion and Biosynthetic Processes in Bacterial Photosynthesis

Abstract

MANY species of photosynthetic bacteria can grow in the absence of molecular oxygen (anaerobically) using organic compounds as the sources of cellular carbon^1,2. These organisms, however, differ strikingly from non-photosynthetic anaerobes in that they seem to be unable to obtain sufficient energy for growth from dark fermentative catabolism of organic substrates. Whatever the carbon source, the photosynthetic bacteria seem to require light for multiplication under anaerobic conditions^2,3. Through the activity of the photochemical system, light energy is converted to chemical energy in the form of adenosine triphosphate (ATP)^4,5, the key fuel for biosynthesis in all types of cells. It is possible that at least part of the energy requirement for anaerobic growth of photosynthetic bacteria can be satisfied by exergonic catabolism of particular substances, but so far this has not been shown. Our search for alternative (dark) anaerobic processes capable of generating ATP at significant rates led us to investigate the intimacy of biochemical coupling between the photochemical energy conversion apparatus and biosynthetic metabolism in the photosynthetic bacterium Rhodopseudomonas capsulatus. The initial aim was to determine the length of the time period over which the biosynthetic machinery can function at a normal rate following cessation of illumination. The experiments reported here indicate a very tight coupling between the light-dependent ATP generating system and overall biosynthesis, and also describe an experimental approach which should prove useful in exploring the general problem of how energy conversion and biosynthetic processes are integrated.