A unique tRNA intron in the variable loop of the extreme thermophile Thermofilum pendens and its possible evolutionary implications.

We describe, for the first time, an intron that is located in the variable loop of a tRNA. It is 18 nucleotides long and occurs within a precursor tRNAGly from the extreme thermophile Thermofilum pendens. The variable loop is less critical for tRNA function than the anticodon arm where other class III introns occur. This raises the possibility that tRNAs with a large variable loop (type II) arose from those with a small variable loop (type I) by retaining a splicing-deficient intron.