Chapter Eight - Role and Evolution of Endogenous Plasmids in Photosynthetic Bacteria

Abstract

Abstract Endogenous plasmids are found in most strains of Rhodobacter sphaeroides , Rhodobacter capsulatus and Rhodospirillum rubrum , but appear to be less common in other species of purple non-sulphur bacteria. They have also been found in some species of green and purple sulphur bacteria. Until recently, there was little conclusive evidence for the functions of these plasmids, and most were considered to be cryptic. The advent of whole genome sequencing has enabled predictions of possible plasmid functions that can then be tested experimentally. In addition, transcriptomic and proteomic studies in R. sphaeroides 2.4.1 have shown that plasmids play an active role in genome function and may code for essential metabolic processes. This chapter summarises experimental and genomic evidence for plasmid function in the anaerobic anoxygenic phototrophs, with particular emphasis on photosynthetic metabolism, nitrogen oxide reduction, cell wall biosynthesis and heavy metal resistance, and new experimental evidence is presented for the role of the endogenous plasmid in the R. capsulatus strains B10 and SB1003. The evolution of these plasmids will also be considered, and insights from nucleotide sequence comparisons will be combined with those from phylogenetic analysis of plasmid replication modules in the Roseobacter clade of the α- Proteobacteria , which includes aerobic anoxygenic phototrophs.