Fig. 2. Dependence of crosslink formation on s⁴UGA programmed ribosomes as a function of eRF1 concentration. Autoradiograph of the crosslink pattern obtained after irradiation of 0.1 µM 5′-end ³²P-labeled s⁴UGA mRNA analog with 0.2 µM reassociated ribosomes and with or without 2 µM tRNA^Asp and variable amounts of eRF1. The irradiated mixtures were analyzed by 10% SDS–PAGE. Lanes 1 and 2 are typical of non-phased and phased ribosome–mRNA complexes. Lanes 3 and 4 show the behavior of the phased ribosomes on the addition of different amounts of eRF1. Molecular mass markers are indicated (in kDa).