The attachment sites of the primary binding proteins L1, L2 and L23 on 23 S ribosomal RNA of Escherichia coli were examined by a chemical and ribonuclease footprinting method using several probes with different specificities. The results show that the sites are confined to localized RNA regions within the large ribonuclease-protected ribonucleoprotein fragments that were characterized earlier. They are as follows:

1. (1) L1 recognizes a tertiary structural motif in domain V centred on two interacting internal loops; the main protein interaction sites occur at the internal loop/helix junctions.2. (2) The L2 site constitutes a single irregular stem/loop structure in the centre of domain IV where non-Watson-Crick pairing is likely to occur.3. (3) L23 recognizes a tertiary structural motif involving a single terminal loop structure and part of an adjacent internal loop at the centre of domain III.

Each of the three primary binding proteins, whose presence is essential for ribosomal assembly, has been associated with important ribosomal functions: L1 lies in the E-site for deacylated tRNA binding while L2 and L23 have been implicated in the P and A substrate sites, respectively, of the peptidyl transferase centre. Moreover, each of the protein sites, but particularly those of L2 and L23, lies at the centre of RNA domains where they can maximally influence both the assembly of secondary binding proteins and the function of the RNA region.