Bacterial clade with the ribosomal RNA operon on a small plasmid rather than the chromosome

Abstract

rRNA is essential for life because of its functional importance in protein synthesis. The rRNA (rrn) operon encoding 16S, 23S, and 5S rRNAs is located on the “main” chromosome in all bacteria documented to date and is frequently used as a marker of chromosomes. Here, our genome analysis of a plant-associated alphaproteobacterium,Aureimonassp. AU20, indicates that this strain has its solerrnoperon on a small (9.4 kb), high-copy-number replicon. We designated this unusual replicon carrying therrnoperon on the background of anrrn-lacking chromosome (RLC) as therrn-plasmid. Four of 12 strains close to AU20 also had this RLC/rrn-plasmid organization. Phylogenetic analysis showed that those strains having the RLC/rrn-plasmid organization represented one clade within the genusAureimonas. Our finding introduces a previously unaddressed viewpoint into studies of genetics, genomics, and evolution in microbiology and biology in general.