Fig. (7) Effect of increasing concentrations of eL42 on the in vitro poly(U)-dependent poly(Phe) synthesis activity of (A) human 80S or (B) E. coli 70S ribosomes. The reaction in 30 µl was started with ribosomes after preincubation for 10 min at 30°C of increasing concentrations of eL42 with charged tRNA^Phe. Activity was determined after 40 min incubation at 37°C in (A) and 10 min incubation at 30°C in (B). For details see Materials and Methods. (C): effect of eL42 addition to the incubation mixture of the poly(U)-dependent poly(Phe) synthesis reaction catalyzed by E. coli 70S ribosomes when [¹⁴C]Phe incorporation has reached a plateau value at time t, prior to the addition of eL42. After 40 min incubation at 37°C (time t), activity was determined with (∆) or without (o) addition of eL42 (20 µM final concentration) in the 70 µl reaction mixture. Activity was determined on 15 µl aliquots withdrawn at time t, t + 5 min, t +10 min, t + 20 min.