INFLUENCE OF MAGNESIUM AND POLYAMINES ON THE REACTIVITY OF INDIVIDUAL RIBOSOMAL-SUBUNIT PROTEINS TO LACTOPEROXIDASE-CATALYZED IODINATION

[Escherichia coli] 30S and 50S subunits, in the presence of either 20 mM Mg2+ or 6 mM Mg2+ and 5 mM spermidine plus 25 mM putrescine, completely associated to form 70S monosomes as monitored by sucrose gradient sedimentation. Subunits maintained under these ionic conditions were compared with 30S and 50S particles at low (6 mM) Mg concentration with respect to the reactivity of individual ribosomal proteins to lactoperoxidase-catalyzed iodination. Altered reactivity to enzymatic iodination of ribosomal proteins S4, S9, S10, S14, S17, S19 and S20 in the small subunit of ribosomal proteins, L2, L9, L11, L27 and L30 in the large subunit following incubation with high Mg or Mg and polyamines suggests that a conformation change in both subunits accompanies the formation of 70S monosomes. The effect of Mg2+ on subunit conformation is mimicked when polyamines are substituted for the Mg necessary for subunit association.