The Genome of Heliobacterium modesticaldum , a Phototrophic Representative of the Firmicutes Containing the Simplest Photosynthetic Apparatus

Abstract

Despite the fact that heliobacteria are the only phototrophic representatives of the bacterial phylum Firmicutes, genomic analyses of these organisms have yet to be reported. Here we describe the complete sequence and analysis of the genome of Heliobacterium modesticaldum, a thermophilic species belonging to this unique group of phototrophs. The genome is a single 3.1-Mb circular chromosome containing 3,138 open reading frames. As suspected from physiological studies of heliobacteria that have failed to show photoautotrophic growth, genes encoding enzymes for known autotrophic pathways in other phototrophic organisms, including ribulose bisphosphate carboxylase (Calvin cycle), citrate lyase (reverse citric acid cycle), and malyl coenzyme A lyase (3-hydroxypropionate pathway), are not present in the H. modesticaldum genome. Thus, heliobacteria appear to be the only known anaerobic anoxygenic phototrophs that are not capable of autotrophy. Although for some cellular activities, such as nitrogen fixation, there is a full complement of genes in H. modesticaldum, other processes, including carbon metabolism and endosporulation, are more genetically streamlined than they are in most other low-G؉C gram-positive bacteria. Moreover, several genes encoding photosynthetic functions in phototrophic purple bacteria are not present in 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.

Both Forward and Reverse TCA Cycles Operate in Green Sulfur Bacteria

“…Alternatively, (Ϫ)-erythro-2-FC may be catalyzed by ACL or/and CS (as discussed below). In contrast, we cannot detect FAc-inhibition in the pyruvate-or acetate-grown photoheterotrophic bacterium Heliobacterium modesticaldum, in which no genes encoding ACL or CS have been annotated (29), and no activity of ACL or CS has been detected (14). Together, two conclusions can be reached with the physiological studies of FAc for C. tepidum.…”

Chlorophyllide a Oxidoreductase Works as One of the Divinyl Reductases Specifically Involved in Bacteriochlorophyll a Biosynthesis

“…Interestingly, filamentous anoxygenic phototrophs Roseiflexus species do not possess either bciA or bciB in the genomes, although these bacteria produce normal 8-ethylated BChl a. Other examples of bacteria lacking DVR genes are BChl b-producing Blastochloris viridis and BChl g-producing Heliobacterium modesticaldum because the C8 vinyl group is directly converted to the C8 ethylidene group in the biosynthetic pathway of these pigments (21)(22)(23).…”

A Genomic Signature and the Identification of New Sporulation Genes

“…However, endospore formers are found in other classes within the Firmicutes and encompass aerobic or anaerobic organisms with a wide range of morphologies, lifestyles, and metabolic traits, including rods, cocci, branching species, plant or animal pathogens or symbionts, syntrophs, sulfate reducers, phototrophs (11)(12)(13)(14)(15)(16)(17), and remarkably, didermic species (18). Despite the extreme diversity of endospore-forming bacteria, the basic architecture of an endospore is conserved across species (19).…”