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

In bacterial genomes, chromosomes are distinguished from plasmids by the localization of essential genes. It has been taken for granted that fundamental genes such as the rRNA (rrn) operon should be transmitted faithfully on the chromosome. Here, we found a striking exception: A plant-associated bacterium, Aureimonas sp. AU20, and its close relatives harbor the rrn operon only on a small, high-copy-number replicon but not on the chromosome. Our findings show the existence of novel genome organization in bacteria.