Molecular Morphology of Ribosomes

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Abstract
Lactoperoxidase‐catalyzed iodination of 30‐S and 50‐S subunits of Escherichia coli ribosomes maintained at 0°C revealed that proteins L2, L5, L10, L11, L15, L26, L27, L29 and L30 of the large subunit and S2, S3, S7, S9, S10, S11, S12, S14, S19 and S21 of the small subunit were the most susceptible to iodination. Unfolding of the 50‐S subunit by the removal of Mg2+ greatly increased iodination of the majority of the large subunit proteins while comparatively few of the 30‐S proteins became more susceptible to iodination under identical conditions. Less extensive conformational changes were observed when the iodination reaction was allowed to proceed at higher temperatures. At 37°C, significant increases in the iodination of proteins S6, S13, S15, S16, L3, L8, L9, L18 and L19 were noted suggesting that a loosening of the ribosome structure occurs at this temperature. Enzymatic iodination had little effect on the activity of the subunits in vitro with respect to G‐factor‐dependent GTPase and poly(U)‐directed poly(phenylalanine)‐synthesizing activities. The lactoperoxidase‐catalyzed iodination system is evaluated as a probe for detecting conformational changes and for determining the topographical nature of the ribosome.