Abstract
Proteobacteria are thought to have diverged from a phototrophic ancestor, according to the scattered distribution of phototrophy throughout the proteobacterial clade, and so the occurrence of numerous closely related phototrophic and chemotrophic microorganisms may be the result of the loss of genes for phototrophy. A widespread form of bacterial phototrophy is based on the photochemical reaction center, encoded by puf and puh operons that typically are in a ‘photosynthesis gene cluster’ (abbreviated as the PGC) with pigment biosynthesis genes. Comparison of two closely related Citromicrobial genomes (98.1% sequence identity of complete 16S rRNA genes), Citromicrobium sp. JL354, which contains two copies of reaction center genes, and Citromicrobium strain JLT1363, which is chemotrophic, revealed evidence for the loss of phototrophic genes. However, evidence of horizontal gene transfer was found in these two bacterial genomes. An incomplete PGC (pufLMC-puhCBA) in strain JL354 was located within an integrating conjugative element, which indicates a potential mechanism for the horizontal transfer of genes for phototrophy.
Highlights
Anoxygenic phototrophic bacteria were proposed to have emerged approximately 3 billion years ago and to be the ancestor of all photosynthetic organisms [1,2,3]
JL354 and the closely-related chemotrophic bacterium Citromicrobium strain JLT1363, we provide evidence that JLT1363 evolved from an Aerobic anoxygenic phototrophic (AAP) bacterium by loss of phototrophy genes to become heterotrophic
Strain JL354 is capable of phototrophic growth on some carbon sources whereas strain JLT1363 is incapable of phototrophy
Summary
Anoxygenic phototrophic bacteria were proposed to have emerged approximately 3 billion years ago and to be the ancestor of all photosynthetic organisms [1,2,3]. Genome sequencing has revealed that AAP bacteria, like purple photosynthetic bacteria, contain a highly conserved ,40 to 50 kb "photosynthesis gene cluster" (PGC) [8]. The genus Citromicrobium is in the Alpha IV AAP bacteria subcluster (Fig. 1A), and it contains only one species, C.
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