Genetic Approaches to The Study of Photosynthetic Prokaryotes

Abstract

Since the advent of life on this planet, photosynthetic prokaryotes have evolved to fill a variety of ecological niches. Many organisms have made novel adaptations in light harvesting mechanisms, pigment compositions, and opportunistic metabolic pathways. We set out to study two such organisms, Acaryochloris marina and Roseobacter denitrificans. The marine cyanobacterium A. marina lives in an environment where much of the photosynthetically active light is absorbed by another cyanobacterial species. Consequently, A. marina utilizes the unique pigment chlorophyll (Chi) d to absorb light that is unused by Chl a and b in the competing species. In spite of the lower-energy light absorbed by Chl d (-30 nm red-shifted) A. marina is able to perform the high-energy demanding oxygen evolution during photosynthesis. In fact, Chl d-containing species closely related to A. marina have been found in a variety of marine environments, though they are always in close proximity to other species. We set out to identify the biosynthetic pathway responsible for constructing Chl d. Using molecular biological methods in concert with bioinformatics, we screened a considerable library of A. marina genes. Although no Chl d synthase was found through these efforts, a precedent was set for studying this relatively new species. The completion of the A. marina genome sequence will facilitate further investigation. R. denitrificans is a model purple aerobic photosynthetic bacterium (APB), a class of organisms that are ubiquitous in euphotic ocean waters. These organisms are the only bacteria that perform photosynthesis in the presence of oxygen, but do not produce oxygen as a result. After completing the genome sequence of R. denitrificans, we characterized its diverse metabolic pathways to identify clues to its lifestyle. Conspicuously missing was the Calvin cycle enzyme ribulose 1,5-bisphosphate carboxylase, the premier enzyme responsible for C02-fixation. We suggest the utility of an alternative mixotrophic C02-fixation pathway using enzymes similar to those in plant C4 carbon-accumulation. The genome of R. denitrificans also hints at tremendous metabolic diversity. The complement of metabolic pathways available to R. denitrificans and other APBs allows them to successfully outcompete organisms with more rigid metabolisms.