Localization of the stringent protein of Escherichia coli on the 50S ribosomal subunit

The "stringent" protein of the ribosome, required for its synthesis of (p)ppGpp, is readily lost during zonal centrifugation. However, enough can be retained to permit its qualitative localization. It is found in native 50S subunits, runoff 70S ribosomes, and polysomes, but not in native 30S subunits. This protein, therefore, appears to be attached to the 50S moiety of the ribosome, and it may be a constant (though easily removed) component of that structure rather than a factor that joins and leaves during the ribosome cycle.