Element and position required. Unusually, the Mycoplasma genitalium enzyme MgR can detect whether this atom is methylated by sensing the difference between North and South pucker. This atom forms cloaked adducts with is·a·toic anhydrides like 1M7 and acyl·imidazoles like NAI. Methylating this heteroatom into an Nm converts a Vaccinia-derived cap-0 into cap-1. Esters on this atom block primer extension during SHAPE probing. The net removal of this atom is accommodated by a thio·redoxin and a tyro·syl radical (-5[1])in the active site of the (15[1])enzyme (*) RNR. This atom (10[1])is the (-5[1])free nucleophile that cyclizes with a phosphate group in a self-cleaving ribozyme. The ability of this heteroatom to hydrolyze phospho·di·ester bonds explains why mRNA vaccines are refrigerated. For 10 points, name this atom that is present in ribose but not deoxyribose. (10[2])■END■ (10[2]0[10])