Identity elements for N-dimethylation of guanosine-26 in yeast tRNAs

N,N-dlmethylguanosine (mJG) is a characteristic nucleoslde that is found in the bend between the dlhydrouridine (D) stem and the anticodon (AC) stem in over 80% of the eukaryotic tRNA species having guanosine at position 26 (G26). However, since a few eukaryotic tRNAs have an unmodified G in that position, G26 is a necessary but not a sufficient condition for dlmethylation. In yeast t R N A ^ G26 is unmodified. We have successively changed the near surroundings of G26 in this tRNA until G26 became modified to m£G by a tRNA(m£G26)methyltransferase in Xenopus laevls oocytes. In this way we have identified the two D-stem basepairs C11-G24, G10-C25 immediately preceding G26 as major identity elements for the dlmethylating enzyme modifying G26. Furthermore, Increasing the extra loop In tRNA*^ from four to the more usual five bases influenced the global structure of the tRNA such that the m£G26 formation was drastically decreased even if the near region of G26 had the two consensus basepairs. We conclude that not only are the two consensus base pairs in the D-stem a prerequisite for G26 modification, but also is any part of the tRNA molecule that influence the 3D-structure important for the recognition between nuclear coded tRNAs and the tRNA(m£G26)methyltransferase.