The site of action of inhibitors of initiation of protein synthesis in reticulocytes

The inhibitors of initiation pactamycin, cycloheximide, NaF, aurintricarboxylic acid and pederine have been found to inhibit the reaction of initiator tRNA with puromycin, as measured by the formation of N-formyl[³⁵S]methionylpuromycin. None of these inhibitors, with the exception of aurintricarboxylic acid, inhibits, however, the binding of the initiator tRNA to ribosomes, as measured by the binding of [³⁵S]Met-tRNA_f to unwashed or washed ribosomes. This binding is stimulated by the codon AUG; the stimulatory activity of this trinucleotide decreases with time even when ribosomes are kept at 0°C. The ribosomal components that bind initiator tRNA have been analyzed by sucrose gradient centrifugation; Met-tRNA_f is bound by polyribosomes, 80-S ribosomes and 40-S ribosomal subunit. Those compounds that inhibit elongation as well as initiation, like cycloheximide, pactamycin and pederine, interfere presumably with a biochemical step common to both processes.