Disposition in The Small Ribosomal Subunit
In the small (30S) subunit of E. coli ribosomes, the proteins denoted S4, S7, S8, S15, S17, S20 bind independently to 16S rRNA. After assembly of these primary binding proteins, S5, S6, S9, S12, S13, S16, S18, and S19 bind to the growing ribosome. These proteins also potentiate the addition of S2, S3, S10, S11, S14, and S21. Protein binding to helical junctions is important for initiating the correct tertiary fold of RNA and to organize the overall structure. Nearly all the proteins contain one or more globular domains. Moreover, nearly all contain long extensions that can contact the RNA in far-reaching regions. Additional stabilization results from the proteins' basic residues, as these neutralize the charge repulsion of the RNA backbone. Protein-protein interactions also exist to hold structure together by electrostatic and hydrogen bonding interactions. Theoretical investigations pointed to correlated effects of protein-binding onto binding affinities during the assembly process
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