We try to mimic the unidirectional sliding-type movement of the PP-tRNA . mRNA complex with respect to the ribosome by looking at the effect of different combinations of cognate tRNAs on the stability of the 70S-AUGUn complex. The association constant for the binary complex 70S-AUGU3 was determined as 6.8 x 10(5) M-1. Addition of tRNAfMet resulted in a 67-fold increase in the association constant, which with both cognate tRNAs is revised to Kassoc = 2.2 x 10(8) M-1. Increasing the chain length of the oligonucleotide from AUGU3 to AUGU13 did not further raise the association constant. The data indicate that the stability of the 70S ribosome . mRNA interaction is governed by the presence of the cognate tRNAs and is topographically restricted to the decoding domains. Since a peptidyl group in the tRNA increases the affinity of AUGU3 for the ribosome by up to 15-fold, we conclude that the affinity of the peptidyl transfer center for the peptidyl moiety pulls the PP-tRNA . mRNA complex from the A (aminoacyl-tRNA) site to the P (peptidyl-tRNA) site. EF-G . GTP or EF-G . GMPPCP 5'-(beta, gamma-methylene)triphosphate] displace tRNAfMet from the quaternary complex 70S . AUGUn . tRNAfMet . tRNAPhe (n = 3 and 6) at Mg2+ less than 25 mM. From the amount of EF-G . GTP bound to a 70S ribosome, it follows that the elongation factor replaces the deacylated tRNA in a stoichiometric way. These data indicate that the EF-G . GTP-dependent release of the deacylated tRNA from the P site, followed by removal of EF-G . GDP from the 50S subunit, is sufficient to trigger the translocation of the mRNA . PP-tRNA complex.

C~0H~3OsN is orthorhombic, space group P2~2~2 I, with a = 25.18, b = 7.127, c --5.715 A, z = 4. The structure was refined to R = 0.049 for 749 counter reflexions. Features of the nucleoside include an orientation of the base at the glycosidic bond N(1)-C(I') in the anti range (53.5°), a ribosyi moiety in the C(2')-endo conformation and a gauche-gauche arrangement of C(4')-C(5'). The packing is characterized by a staggered arrangement of the bases and a clear separation of hydrophobic and hydrophilic regions.Comparison with other pyrimidine nucleosides suggests an interrelationship between the puckering of the ribose and the orientation of the base, which has its structural basis in the formation of the intramolecular interaction C(6)-H-.. 0(5').The secondary and tertiary structures of nucleic acids are mainly determined by the stereochemistry of the monomeric building blocks. The influence of modifications in the base moiety on the stereochemistry of nucleic acids has not yet been clarified. Therefore, as part of our studies on the structures of pyrimidine nucleosides, some of which occur in tRNA, we have solved the crystal structure of 3-deaza-4-deoxyuridine (1-fl-o-ribofuranosyl-2-pyridone), which is the least polar nucleoside of this series. This nucleoside, furthermore, represents a model compound, which shows the space filling of a pyrimidine nucleoside, but has no ability to form Watson-Crick-type hydrogen bonds with adenosine.

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CllHtTN30 6 is monoclinic, space group C2, with a = 16.224(6), b = 8.349(3), c = 10.347 (4) A, fl = 111.18 (2) °, Z = 4. The structure, which was refined to R = 0.058 for 1245 counter reflections, exhibits the influence of an electron-donating 5-substituent on the nucleoside conformation, supporting the results from the structure of 5-aminouridine. The orientation of the base is anti (X = 51"0°); the ribosyl moiety shows a C(2')-endo conformation and a gauche-trans arrangement of the C(5')-O(5') bond. There is no base stacking between adjacent layers parallel to the ac plane. Two molecules are connected to each other by their 2-keto and 3-imino groups. These U:U base-pairs are discussed with respect to other nucleosides like 5-0567-7408/79/040920-04501.00 methoxyuridine which amplify the wobble recognition in tRNA's.