Specific cross-linking of proteins S7 and L4 to ribosomal RNA, by UV irradiation of Escherichia coli ribosomal subunits.

Radioactive 30S and 50S subunits from E. coli ribosomes were irradiated with UV light, under conditions giving rise to approximately 10% cross-linking of protein to ribosomal RNA. Irradiation to levels of cross-linking higher than 10% caused unfolding of the ribosomal subunits, even in the presence of 5 mM magnesium. The specificity of the cross-linking reaction at this low level was found to be extremely high. Cross-linked RNA-protein complexes, freed from unbound protein, were treated with nuclease and then analysed on Sarkosyl gels. S7 was found to be the primary target of the cross-linking reaction in the 30S particle. This was proven by using subunits from both E. coli MRE 600 and A19, whose respective S7 species differ markedly. In the 50S particle, L4 was the primary target, although L2 was also cross-linked to a small extent. Ambiguity in the identification of L4 in the Sarkosyl system was resolved by two-dimensional electrophoresis, which was also used to demonstrate a genuine linkage to RNA in the case of both S7 and L4; proteins spots containing 32P were observed, derived from these two proteins, when subunits containing 32P-RNA were irradiated, treated with nuclease, and applied to the electrophoresis. The identities of S7, L4 and L2 were finally confirmed by Ouchterlony tests with protein-specific anti-sera.