The photomicrograph, kindly provided by Caroline Harwood (University of Iowa, USA) shows Rhodopseudomonas palustris cells.

Rhodopseudomonas palustris is a purple photosynthetic bacterium that can be isolated from many diverse environmental locations. The key to success for this microorganism is its extraordinary metabolic versatility — R. palustris can exploit light, inorganic compounds and organic compounds for energy. It can also acquire carbon from organic componds and by CO2 fixation, grow with or without oxygen, and can fix nitrogen. Now, with the publication of the R. palustris genome in Nature Biotechnology, an important step has been taken towards understanding how this bacterium coordinates and expresses its many metabolic capabilities in response to changing environmental conditions.

The genome of R. palustris comprises a chromosome 5.46 Mb in size and a 8.4-kb circular plasmid. The sequence reveals the presence of a large number of genes that allow the microorganism to benefit from considerable flexibility within a given type of metabolism and to take advantage of changes in available carbon, nitrogen, light and oxygen. Genes for three different nitrogenases, multiple aromatic degradation pathways, four light harvesting (LH)-2 complexes and multiple oxidoreductases were discovered. The genome of R. palustris also encodes about 325 transport systems — amounting to more than 700 genes — which adds up to almost 15% of the genome devoted to this activity. In other bacterial genomes, typically 5–6% of a genome consists of transport genes, indicating that R. palustris is richly equipped to sense and acquire diverse compounds from its environment. Of course, with so many metabolic options, it is crucial that the bacterium can sense changing environmental conditions rapidly and regulate gene expression appropriately for survival and growth. To this end, R. palustris devotes 9.3% of its genome to regulation (versus 5–6% for most free-living bacteria), including 63 signal transduction kinases and 79 response regulator receiver domains.

The availability of the R. palustris genome provides the framework that will allow researchers to explore the regulatory strategies employed by the microorganism to select and integrate its many metabolic capabilities. By understanding, and ultimately manipulating these processes, the full biotechnological potential of R. palustris can be realized, including the development of the bacterium as a biocatalyst for hydrogen production, and devising strategies to maximize both its carbon-recycling capabilities and its promise as an effective aid to environmental restoration through microbially mediated bioremediation.