A factor in serum and amniotic fluid is a substrate for the tRNA-modifying enzyme tRNA-guanine transferase.

Q factor, a substance found in animal serum that enables cultured mammalian cells (L-M) to produce tRNA containing queuine (the base of "nucleoside Q", queuosine), has been purified to homogeneity from bovine amniotic fluid. Q factor causes the appearance of Q-containing tRNAAsp in the L-M cells cultivated in serum-free medium, and this was used as an assay to monitor the purification of Q factor. Q factor is a competitive inhibitor of guanine for rabbit reticulocyte tRNA-guanine trnsferase, with a K1 of 4.5 x 10(-8) M. Q factor is inactivated in both the L-M cell and tRNA-guanine transferase assays by treatment with periodate or cyanogen bromide, both of which react with queuine. In L-M cells, nearly complete conversion of Q-free to Q-containing tRNAAsp is observed within 24 hr after addition of pure Q factor to the medium; actinomycin D, cycloheximide, and cycloleucine, inhibitors of RNA synthesis, protein synthesis, and nucleic acid methylation, respectively, do not inhibit this conversion. The product of the reaction, catalyzed by pure rabbit reticulocyte tRNA-guanine transferase, between Q factor and rabbit reticulocyte Q-free tRNAHis is chromatographyically indistinguishable from Q-containing tRNAHis.