Ribosomal protein S1 associates with Escherichia coli ribosomal 30-S subunit by means of protein-protein interactions.

Ribosomal proteins S1 when associated with the 30-S subunit does not interact with 16-S RNA but its binding is determined mostly by protein-protein interactions. These conclusions are based on the following data. 1. Ultraviolet irradiation (lambda = 254 nm) of the 30-S subunit does not result in the covalent cross-linking of S1 with 16-S RNA at irradiation doses up to 150 quanta/nucleotide, whereas the irradiation under the same conditions of S1 . polynucleotide complexes [S1 . poly(U), S1 . poly(A) and S1 . Q beta phage RNA] induces effective formation of polynucleotide-protein cross-links. 2. Mild treatment of 30-S subunits lacking S-1 with RNase A or with cobra venom endonuclease results in removal of 10--20% of the total nucleotide material but does not affect their sedimentation characteristics of their S1 binding capacity. 3. The association of S1 with S1-depleted 30-S subunits is insensitive to aurintricarboxylic acid, which is known as a strong inhibitor of complex formation between S1 and polynucleotides. 4. Mild trypsin treatment of S1-depleted 30-S subunits greatly reduces their S1 binding capacity.