LOCALIZATION OF 2 DOMAINS OF A MUTANT FORM OF ESCHERICHIA-COLI PROTEIN L7 L12 THAT BINDS THE LARGE RIBOSOMAL-SUBUNIT AS A SINGLE DIMER/

Escherichia coli ribosomal protein L7/L12 occurs on the large subunit as two dimers: one dimer is extended and comprises the stak, while the second dimer is folded and occupies a site on the subunit body. A var iant protein, in which all 18 amino acids of the flexible hinge region that links separate N-terminal and C-terminal domains of L7/L12 has b een deleted, binds the subunit as a single dimer and does not generate stalks that are visible in electron micrographs. Monoclonal antibodie s directed against each domain of the protein have been used to locali ze the variant in electron micrographs of 50S subunits. Both C-termina l domains are seen at a shoulder of the subunit, near its edge as view ed in the most common quasisymmetric projection. N-terminal domains ar e placed on the subunit body, about 50 Angstrom from the C-terminal do mains. The antibody to the N-terminal domain also causes dissociation of the variant dimer from the particle and the formation of oligomeric antibody-protein dimer complexes. Similar complexes were seen previou sly (Olson HM et al (1986) J Biol Chem 261, 6924-6936) when this antib ody induced dissociation of one dimer of the native protein. We conclu de that the shortened variant most probably occupies the lower-affinit y site on the subunit that is normally filled by the stalk dimer.