title: Relaxation kinetics of E. coli ribosomes: evidence for the reaction of 30S . IF3 complex with 50S ribosomal subunits.
abstract: Addition of initiation factor IF3 to solutions of E. coli ribosomes dramatically alters their behavior in pressure-jump relaxation kinetic experiments in which 90 degrees light-scattering is used to monitor the macromolecular reaction. The effect of IF3 on relaxation processes attributed to "tight" couples is strongly dependent on the Mg2+ concentration. At 2.5 mM Mg2+, addition of 1 molar equivalent of IF3 decreases the relaxation amplitude by a factor of 3 relative to ribosome solutions without IF3. However, at 5.0 mM Mg2+, addition of 1 molar equivalent of IF3 produces a marked increase in the relaxation amplitude, by a factor of 2-8 fold relative to ribosomes in the absence of IF3. IF3 has no effect on the relaxation process attributed to "loose" couples at 10 mM Mg2+. While we are unable to propose a precise mechanism for IF3 action with the data on hand, our results require that the 30S . IF3 complex either reacts with the 50S subunit, forming a 70S . IF3 intermediate, or acts as a pool of reactive 30S subunit. Further kinetic evidence is required to distinguish between these possible pathways.
mesh_terms: D004926:Escherichia coli / Q000378:metabolism*; D007700:Kinetics; D008027:Light; D008274:Magnesium / Q000494:pharmacology; D010442:Peptide Chain Initiation, Translational; D010448:Peptide Initiation Factors / Q000378:metabolism*; D011312:Pressure; D012270:Ribosomes / Q000187:drug effects / Q000378:metabolism*; D012542:Scattering, Radiation
chemical_list: D010448:Peptide Initiation Factors; D008274:Magnesium