The backwards tRNA polymerase, Thg1

Post date: Nov 17, 2016 5:42:26 PM

Today I learned of a widely conserved enzyme in eukarya, tRNA-His guanlyltransferase (Thg1), whose job is to add a guanosine to the 5' end of tRNA-His. Thg1 is weird: it was the first discovered polymerase with 3' to 5' polymerization activity. The G is thought to ensure specificity of the addition of His to the tRNA, and is found in all branches of life. In yeast, Thg1 is essential. You might ask, why not encode the G in the genome? Good question, after all, that's what most bacteria do. But this solution requires additional mutations to keep the tRNA functional. (Or you might ask, why not evolve a tRNA-His aminoacylase that doesn't require the G? Good question, that's what trypanosomes have done.) Meanwile, in most eukaryotes, Thg1 is still happily adding a single G to a single tRNA, in a unique backwards direction.