We have used rapid probing methods to follow the changes in reactivity of residues in 16 S rRNA to chemical and enzymatic probes as ribosomal proteins S2, S3, S10, S13 and S14 are assembled into 30 S subunits. Effects observed are confined to the 3′ major domain of the RNA and comprise three general classes. 1. (1) Monospecific effects, which are attributable to a single protein. Proteins S13 and S14 each affect the reactivities of different residues which are adjacent to regions previously found protected by S19, S10 effects are located in two separate regions of the domain, the 1120 1150 stem and the 1280 loop; both of these regions are near nucleotides previously found protected by S9. Both S2 and S3 protect different nucleotides between positions 1070 and 1112. In addition, S2 protects residues in the 1160 1170 stem-loop. 2. (2) Co-operative effects, which include residues dependent on the simultaneous presence of both proteins S2 and S3 for their reactivities to appear similar to those observed in native 30 S subunits. 3. (3) Polyspecific effects, where proteins S3 and S2 independently afford the same protection and enhancement pattern in three distal regions of the domain: the 960 stem-loop, the 1050 1200 stem and in the upper part of the domain (nueleotides 1070 to 1190). Proteins S14 and S10 also weakly affect the reactivities of several residues in these regions. We believe that several of the protected residues of the first class are likely sites for protein-RNA contact while the third class is indicative of conformational rearrangement in the RNA during assembly. These results, in combination with the results from our previous study of proteins S7, S9 and S19, are discussed in terms of the assembly, topography and involvement in ribosomal function of the 3′ major domain.
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