Katze J R, Farkas W R
Proc Natl Acad Sci U S A. 1979 Jul;76(7):3271-5. doi: 10.1073/pnas.76.7.3271.
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.
Q因子是一种存在于动物血清中的物质，它能使培养的哺乳动物细胞（L-M细胞）产生含有queuine（“核苷Q”即queuosine的碱基）的tRNA。Q因子已从牛羊水纯化至同质状态。Q因子能使在无血清培养基中培养的L-M细胞中出现含Q的天冬氨酸tRNA，这被用作监测Q因子纯化的一种测定方法。Q因子是鸟嘌呤对兔网织红细胞tRNA-鸟嘌呤转移酶的竞争性抑制剂，其K1为4.5×10^(-8)M。在用高碘酸盐或溴化氰处理时，Q因子在L-M细胞和tRNA-鸟嘌呤转移酶测定中均被灭活，这两种试剂都与queuine发生反应。在L-M细胞中，向培养基中添加纯Q因子后24小时内可观察到无Q的天冬氨酸tRNA几乎完全转化为含Q的天冬氨酸tRNA；分别作为RNA合成、蛋白质合成和核酸甲基化抑制剂的放线菌素D、环己酰亚胺和环亮氨酸并不抑制这种转化。由纯兔网织红细胞tRNA-鸟嘌呤转移酶催化的Q因子与兔网织红细胞无Q的组氨酸tRNA之间反应的产物，在色谱上与含Q的组氨酸tRNA无法区分。
