The birth of a bacterial tRNA gene

While the major function of transfer RNA is conserved across the tree of life, organisms differ in the types and numbers of tRNA genes that they carry. The evolutionary mechanisms behind the emergence of different tRNA gene sets remain largely obscure. Here, we report the rapid and repeated evolution of a tRNA gene set in laboratory populations of the bacterium SBW25. Deletion of the non-essential, single-copy tRNA gene from SBW25 results in a sub-optimal tRNA gene set. Compensation occurs within 35 generations large (45-290 kb), direct, tandem duplications in the chromosome. Each duplication contains a gene, and is accompanied by a two-fold increase in tRNA-Ser(UGA) in the mature tRNA pool. This work demonstrates that the composition of tRNA gene sets – and mature tRNA pools – can readily evolve by duplication of existing tRNA genes, a phenomenon that could explain the presence of multiple identical tRNA gene copies within genomes.