Transfer ribonucleic acids from Escherichia coli treated with 5-fluorouracil

5-Fluorouracil (FU) is readily incorporated into the transfer ribonucleic acids (tRNAs) of Escherichia coli. tRNA synthesized in the presence of the analog may have up to 100% replacement of uracil by FU. The relative amounts of the other three major bases are unchanged but the relative amounts of ribothymidylic and pseudouridylic acids are reduced. Incorporation of FU into tRNA does not affect its ability to accept amino acids or to transfer phenylalanine into polyphenylalanine or lysine into polylysine in an E. coli S-30 system. Transfer of lysine into polypeptides synthesized under the direction of bacteriophage R17 RNA is inhibited by tRNA from FU-treated cells. This inhibition does not appear to be due to errors in codon recognition by lysyl-tRNA containing FU. There was no difference in response to a series of A,G copolymers between lysyl-tRNA containing FU and control lysyl-tRNA in the ribosome binding assay. tRNA containing FU has been separated from contaminating unsubstituted tRNA by chromatography on columns of methylated albumin kieselguhr. tRNA containing FU has been shown to possess an altered secondary structure as judged by its thermal denaturation profile.