Fig. (5) Peptidyl transferase activity of the wild-type or mutant E. coli 70S ribosomes. Peptidyl transferase activity was determined as the extent of puromycin that has reacted with the “Ac-[³H]Phe-tRNA.poly(U).reconstituted ribosome” complex, in the absence or the presence of the elongation factor EF-G (Materials and Methods).