Immunoelectron microscopic localization of the site of photo-induced affinity labeling of the small ribosomal subunit with puromycin.

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Abstract
Ribosomes from Escherichia coli strain TPR201 (which lack N6,N6-dimethyladenosine) were photoaffinity labeled with [3H]puromycin in the presence of chloramphenicol. Puromycin-modified 30S ribosomal subunits appear to be identical to untreated subunits in electron micrographs and are efficiently precipitated by antibodies to the puromycin analog N6,N6-dimethyladenosine. Electron micrographs of subunit-antibody complexes show ribosomal subunits to which an individual antibody molecule is bound and pairs of 30S subunits which appear to be crosslinked by a single Ig[immunoglobulin]G molecule. A predominant site of puromycin photoaffinity labeling was identified from the apparent point of contact of antibody and ribosomal subunit. The puromycin site is localized to the small upper portion of the particle on the side opposite to the subunit platform. This location is close to that reported for ribosomal protein S14, the major puromycin-labeled protein in the small ribosomal subunit.