Initiator proteins for the assembly of the 50S subunit from Escherichia coli ribosomes.

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Abstract
An rRNA-binding protein that binds to the rRNA independently of other proteins during the course of ribosomal assembly is termed assembly initiator protein. In spite of the large number of rRNA-binding proteins (more than 17 out of 32 proteins have been identified in the case of the large ribosomal subunit), only a very small number of proteins should actually initiate ribosomal assembly for theoretical reasons. Only 2 of the L proteins derived from the large subunit (50S) [of E. coli ribosomes] function as assembly initiator proteins. Two different techniques are used to identify these initiator proteins: reconstitution experiments with purified proteins and pulse-chase experiments during in vitro assembly. Both methods independently identify L24 and L3 as initiator proteins for the 50S assembly. The existence of 2 initiator proteins (not just 1) resolves an apparent contradiction, i.e., that rRNA is synthesized in excess under unfavorable growth conditions, yet rRNA-binding proteins should be available for translational control.