Biosynthetic Production of [N²,1,3,7,9ó¹⁵N]Guanosine and [1,3,7,9-¹⁵N]Inosine and Conversion into [N⁶,1,3,7,9ó¹⁵N]Adenosine for Structure Elucidation of RNA by Heteronuclear NMR   A procedure was developed for the biosynthetic preparation of ¹⁵N-labelled guanosine and inosine through the action of a mutant Bacillus subtilis strain. Crude (¹⁵N₅)guanosine and (¹⁵N₄)inosine were isolated from the culture filtrate by precipitation and anion-exchange chromatography. No cell lysis and no enzymatic degradation was necessary. The per-isobutyrylated derivatives were isolated from a complex mixture, purified by virtue of their different lipophilicity, and separated in three steps involving normal and reversed-phase silica-gel chromatography. One litre of complex nutrient medium yielded 8.44 mmol of guanosine derivative and 2.84 mmol of inosine derivative with an average ¹⁵N enrichment of 83.5 and 91.9 atom-%, resp. (¹⁵N₅)Adenosine was obtained from 2',3',5'-tri-O-isobutyryl (¹⁵N₄)inosine through the ammonolysis of its 1,2,4-triazolyl derivative with aqueous ¹⁵NH₃.