Localization of Escherichia coli ribosomal protein S4 on the surface of the 30S ribosomal subunit by immuno electron microscopy

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Abstract
The location of the ribosomal protein S4 on the surface of the 30S subunit of E. coli ribosomes was determined by immuno electron microscopy. Immunoglobulins from six separate S4-specific antisera were investigated. In accordance with earlier findings protein S4 was shown to have an elongated conformation within the intact ribosome (Lake et al., 1974; Stöffler and Tischendorf, 1975; Tischendorf et al., 1975). S4-specific antibodies bind at four distinct sites on 30S ribosomal subunits, designated A, B, C and D. Two antibody binding sites (A and B) are located on the “head”, they were shown to be separated by 70–80 Å. The distance between these sites and the two sites C and D on the “body” of the subunit amounts to at least 90–125 Å; hence protein S4 should be extended to a total length of approximately 160–200 Å. At least three of the four S4-specific antibody binding sites were observed with antibodies from each of the six antisera investigated. These sites were observed independently of whether isolated S4 protein, an S4·16S rRNA complex or 30S ribosomes were used as the antigen. They could also be visualized with monovalent antibody fragments (Fab). S4-specific antibodies enriched by affinity chromatography bound at identical sites which conclusively ensures that antibody binding at these sites is specific for protein S4 and is not due to the presence of contaminating antibodies in the antisera. Protein S4 has therefore definitely an extended or fibrous shape in the intact ribosome. The implications of these results with respect of the correlation of each of the sites to their respective antigenically active fragments within protein S4 are discussed.