The L7/L12 stalk, a conserved feature of the prokaryotic ribosome, is attached to the large subunit through its N terminus

Abstract The L7/L12 stalk, a distinctive feature of the large ribosomal subunit from Escherichia coli , is shown to be a general morphological feature associated with large subunits from representative prokaryotes. In the gram-positive thermophile Bacillus stearothermophilus , the stalk consists of four copies of ribosomal proteins B.L13, the functional and amino acid sequence homologue of E. coli proteins L7/L12. That this protein comprises the stalk is shown by electron microscopy of 50 S subunits that have had B.L13 selectively removed, and by immune electron microscopic mapping of B.L13. In addition, a fragment consisting approximately of the N-terminal one-third of B.L13, known to compete with B.L13 for binding to B.L13-depleted 50 S cores, is shown to be located in the region of the stalk proximal to the body of the 50 S subunit, and the C-terminal two-thirds of B.L13 is shown to be located in the globular region of the stalk that is distal to the large subunit. Taken together these results suggest that in the isolated subunit at least three and probably all four chains of B.L13, and of the corresponding L7/L12 proteins in other prokaryotes, are arranged with the N termini of the proteins contacting the body of the 50 S subunit and with the C-terminal globular regions of the protein extending away from the subunit into the cytoplasm.