The sarcin–ricin loop of 23S rRNA is essential for assembly of the functional core of the 50S ribosomal subunit

ΔSRL 50S subunits bind P-site tRNA to WT levels, but are defective in the puromycin reaction. (A) 50S subunit stimulation of P-site tRNA binding to 30S subunits (black bars) and puromycin reactivity (gray bars) were assayed in complexes containing [³⁵S]-fMet-tRNA^fMet, mRNA, and 30S subunits formed in the presence or absence of WT or ΔSRL 50S subunits as indicated. (B) ΔSRL (▲) and WT (■) 50S subunits bound to [³⁵S]-fMet-tRNA^fMet were reacted with increasing concentrations of puromycin. The results were fit to a single exponential curve and are also shown as a double-reciprocal plot (inset).