Catalyzes the NADPH-dependent reduction of 7-cyano-7-deazaguanine (preQ0) to 7-aminomethyl-7-deazaguanine (preQ1), a late step in the queuosine pathway. Is highly specific for its natural substrate preQ0, since it cannot use various aliphatic, aromatic and heterocyclic nitriles, although it can reduce the substrate analog 5-cyanopyrrolo[2,3-d]pyrimidin-4-one with lesser efficiency. 7-aminomethyl-7-carbaguanine + 2 NADP(+) = 7-cyano-7-deazaguanine + 3 H(+) + 2 NADPH Is totally inhibited by 4-aminobenzylcyanide in vitro. kcat is 3.9 min(-1) for the reduction of 7-cyano-7-deazaguanine, and 9.3 min(-1) for that of 5-cyanopyrrolo[2,3-d]pyrimidin-4-one (at 55 degrees Celsius). Optimum pH is 7.5. Enzymatic activity increases 12-fold in response to a change in temperature from 25 to 65 degrees Celsius. Displays a half life of 43 hours at 55 degrees Celsius. tRNA modification; tRNA-queuosine biosynthesis. Belongs to the GTP cyclohydrolase I family. QueF type 1 subfamily.