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, benzylic and heterocyclic nitriles, such as acetonitrile, benzonitrile, benzylcyanide and 2-cyanopyrrole, 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 KM for preQ0 appears to be inferior to 1.5 uM, and kcat is 0.1268 sec(-1) (PubMed:23410922). kcat is 6.5 min(-1) with preQ0 as substrate and 3.6 min(-1) with 5-cyanopyrrolo[2,3-d]pyrimidin-4-one as substrate (PubMed:23595998). Optimum pH is 7. Retains less than 20% of activity at pH 9 (PubMed:23410922). Optimum temperature is 37 degrees Celsius. Displays a half life of 28.2 hours and 12.8 hours at 37 and 40 degrees Celsius, respectively, but at 50 degrees Celsius the half life time drops to 6 minutes (PubMed:23410922). tRNA modification; tRNA-queuosine biosynthesis. Homodimer. Belongs to the GTP cyclohydrolase I family. QueF type 2 subfamily.