Alternate names include delta(2)-isopentenylpyrophosphate transferase, IPP transferase, 2-methylthio-N6-isopentyladenosine tRNA modification enzyme. Catalyzes the first step in the modification of an adenosine near the anticodon to 2-methylthio-N6-isopentyladenosine. Understanding of substrate specificity has changed. [Protein synthesis, tRNA and rRNA base modification]