Figure 5.

Proposed models for the development of permuted tRNA genes. (A) Permuted tRNA genes might have formed by gene duplication and loss of the outer segments. A tandem repeat of an intron-containing tRNA gene could be arranged into a permuted tRNA gene by the accumulation of mutations at both ends. Alternative models are based on reverse transcription of permuted or circularized pre-tRNA molecules. (B) A permuted pre-tRNA formed by an interaction between the 3′-half of the tRNA encoded by the initial gene and the 5′-half of the duplicated tRNA derived from the tandemly-repeated gene may have been reverse transcribed from the point indicated by an arrowhead and integrated back into the genome. (C) A circular intron-containing pre-tRNA produced by ligation of an intron-containing pre-tRNA at the acceptor-stem may also have been reverse transcribed and integrated into the genome.