Chloroplast ribosome structure. Electron microscopy of ribosomal subunits and localization of N6,N6-dimethyladenosine by immunoelectronmicroscopy.

Ribosomal subunits from the chloroplasts of Alaskan peas have been studied by immunoelectronmicroscopy. Electron micrographs of negatively stained small and large ribosomal subunits show particles of similar size and in the same characteristic projections described for the ribosomal subunits of Escherichia coli (Lake, J. A. (1976) J. Mol. Biol. 105, 131-159), although minor structural differences are apparent. High pressure liquid chromatographic analysis shows the modified nucleoside N6,N6-dimethyladenosine is conserved in chloroplast 16 S ribosomal RNA, presumably as two successive residues near the 3' end. Antibodies directed against N6,N6-dimethyladenosine were allowed to react with chloroplast 30S ribosomal subunits. Electron microscopy showed individual subunit-antibody complexes and pairs of ribosomal subunits cross-linked by a single antibody. In 94% of the complexes observed, antibody contact was consistent with a dimethyladenosine localization near the end of the small subunit platform, in an area of subunit contact in the 70 S ribosome. This localization is analogous to the placement of N6,N6-dimethyladenosine in the E. coli ribosome (Politz, S. M., and Glitz, D. G. (1977) Proc. Natl. Acad. Sci. U. S. A. 74, 1468-1472).