Assembly of the 30S Ribosomal Subunit

Protein synthesis involves nearly a third of the total molecules in a typical bacterial cell. Within the cell, protein synthesis is performed by the ribosomes, and research over several decades has investigated ribosomal formation, structure, and function. This review provides an overview of the current understanding of the assembly of the Escherichia coli 30S ribosomal subunit. The E. coli 30S subunit contains one rRNA molecule (16S) and 21 ribosomal proteins (r-proteins; S1 to S21). The formation of functional subunits can occur as a self-assembly process in vitro; i.e., all the information required for the formation of active ribosomes resides in the primary sequences of the r-proteins and rRNAs. In vitro reconstitution of functional 30S subunits is carried out by using a mixture of TP30, individually purified natural or recombinant r-proteins, and natural 16S rRNA. Chemical probing and primer extension analysis have been used extensively to monitor changes in the reactivities of nucleotides in 16S rRNA during the in vitro reconstitution of 30S subunits. The potential roles for r-proteins in 30S subunit assembly were determined by omitting single proteins in reconstitution experiments. The RNPs resulting from single protein omissions were examined in terms of their composition and function to determine the roles of the absent proteins. Recent developments in understanding the structure of the 30S subunit have led to speculation about roles for some of the r-proteins in assembly. The crystal structures of the 30S subunit (1, 2) and the 70S ribosome (3) reveal details of the r-protein and rRNA interactions.