The genome of Heliobacterium modesticaldum, a phototrophic representative of the Firmicutes containing the simplest photosynthetic apparatus

Department of Biology, Washington University, Campus Box 1137, St. Louis, MO 63130; Department of Microbiology, Southern Illinois University, Carbondale, IL 62901; Institute of Low Temperature Science, Hokkaido University, N19W8, Sapporo 060-0819, Japan; Translational Genomics Research Institute, Scottsdale, AZ 85259; School of Natural Sciences, University of California, Merced, CA 95344; School of Biological Sciences (A08), The University of Sydney, NSW 2006, Australia; Departments of Biology and Chemistry, Washington University, Campus Box 1137, St. Louis, MO 63130; School of Life Sciences, Arizona State University, Tempe, AZ 85287

^* To whom correspondence should be addressed. Email: j.touchman{at}asu.edu.

	Abstract

Despite being the only phototrophic representatives of the bacterial phylum Firmicutes, genomic analyses of heliobacteria have yet to be reported. Here we present the complete sequence and analysis of the genome of Heliobacterium modesticaldum, a thermophilic species of this unique family of phototrophs. The genome is a single 3.1 Mb circular chromosome containing 3138 open reading frames. As suspected from physiological studies of heliobacteria that have failed to show photoautotrophic growth, genes encoding enzymes of known autotrophic pathways in other phototrophic organisms, including ribulose bisphosphate carboxylase (RuBisCO—Calvin cycle), citrate lyase (reverse citric acid cycle), and malyl-CoA lyase (3-hydroxypropionate pathway), are not present in the H. modesticaldum genome. Thus, heliobacteria appear to be the only known anaerobic anoxygenic phototrophs incapable of autotrophy. Although some cellular activities, such as nitrogen fixation, have a full complement of genes in H. modesticaldum, other processes, including carbon metabolism and endosporulation, are more genetically streamlined than in most other low G+C Gram-positive bacteria. Moreover, several genes encoding photosynthetic functions in phototrophic purple bacteria are absent from the heliobacteria. In contrast to the nutritional flexibility of many anoxygenic phototrophs, the complete genome sequence of H. modesticaldum reveals an organism with a notable degree of metabolic specialization and genomic reduction.