| Abstrakt: |
The interaction between mRNA and Escherichia coliribosomes has been studied by photochemical cross-linking using mRNA analogues that contain 4-thiouridine (s4U) or s4U modified with azidophenylacyl bromide (APAB), either two nucleotides upstream or eight nucleotides downstream from the nucleotide sequence ACC, the codon for tRNAThr. The sequences of the mRNA analogues were described earlier (Stade, K., Rinke-Appel, J., and Brimacombe, R.(1989) Nucleic Acids Res.17, 9889-9908; Rinke-Appel, J., Stade, K., and Brimacombe, R.(1991) EMBO J.10, 2195-2202). Under equilibrium conditions, both of these mRNA analogues bind and cross-link to 70 S ribosomes without the presence of tRNAThr; however, there are significant increases both in binding and particularly in cross-linking in the presence of the tRNAThr. Four regions contain cross-linking sites that increase in the presence of tRNA, C1395, A532, A1196(and minor sites around these three positions), and C1533/U1532. Three other cross-linking sites, U723, A845, and U1381, show very little change in extent of cross-linking when tRNA is present. A conformational change in the 30 S subunit allowing additional accessibility to the 16 S rRNA by the mRNA analogues upon tRNA binding best explains the behavior of the tRNA-dependent and tRNA-independent mRNA-16 S rRNA cross-linking sites. |
