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

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. Here we demonstrate that only two of the L proteins derived from the large subunit (50S) 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 two initiator proteins (not just one) resolves an apparent contradiction--namely, that on the one hand, rRNA is synthesized in excess under unfavorable growth conditions, whereas on the other hand, rRNA-binding proteins should be available for translational control.