Escherichia coli formylmethionine transfer RNA has been modified by reaction with N-a~etoxy-9-[~~C]acetylaminofluorene, a drug which is known to cause the covalent attachment of the acetylaminofluorene group to the 8 position of guanosine residues. After separation of radioactive from nonradioactive molecules, the modified tRNA was cleaved by TI or pancreatic ribonuclease. The localization of the modified guanosine residues in the resulting oligonucleotides was determined after DEAE-Sephadex chromatography. The A lthough the primary structure of tRNA and its function in protein synthesis are now reasonably well understood, the relationship between its structure and biological functions remains to be elucidated. Chemical modification of tRNA can provide useful information, not only about the specific structural requirements for multiple functions of tRNA, but also about the three-dimensional structure of tRNA molecules (see reviews: von der Haar et al., 1971 ;Chambers, 1971). Miller and coworkers (1966) have established that 2-acetylaminofluorene, a potent carcinogen which is covalently bound to liver tRNA when administered in uiuo (Henshaw and Hiatt, 1963 ;Agarwal and Weinstein, 1970), requires metabolic activation as a prerequisite for complexing with nucleic acids. The final metabolite for this interaction is an ester of N-hydroxy-AAF. With a synthetically prepared ester, Nacetoxy-AAF, it is possible to directly bind AAF to the 8 position of G residues in nucleic acids at neutral pH in vitro, thereby producing the same type of modification as that which occurs in vivo (Miller et al., 1966;Kriek et al., 1967). In preceding papers in this series we have described that this modification causes major changes in the conformational properties of oligonucleotides which include rotation of the guanine base about the glycosidic linkage and the intramolecular stacking of fluorene with an adjacent base (Nelson et al., 1971). These results provided essential chemical background for the use of N-acetoxy-AAF for the modification of a specific tRNA whose primary structure is known. Previously, a similar approach was used in this laboratory for the in vitro modifica-
Source of material: Stoichiometric amounts of CuS and VsSg were finely grinded. The homogenized mixture was fired at 873 К for 8 hours under sulfur atmosphere. This product was then annealed at 1173 К for 12 hours. In contrast to reference 1, no unusual electron density around sulphur was observed in a 3-D difference map.
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