Fig. 5: Evolutionary model of bacteria without chromosomal rrn operons based on the findings of in this study.
From: Bacteria can maintain rRNA operons solely on plasmids for hundreds of millions of years
(1) An ancestral bacterium obtains a prerequisite Rep_3-type plasmid by horizontal gene transfer. (2) An rrn operon is translocated to the Rep_3-type plasmid. (3) Effective copy numbers of rrn operons increase on the high copy number Rep_3-type plasmid. (4) Resultant evolutionary pressure leads to loss of chromosomal rrn operons, making the rrn plasmid indispensable to the bacteria. (5) Copy numbers of rrn plasmids decrease by keeping effective copy numbers of rrn operons, the rrn plasmid acquires a partitioning system for stable inheritance, and chromosomal tRNA genes undergo tandem duplications.
