We have measured the binding isotherms of C‐A‐C‐C‐A(3′NH)‐[14C]Phe to the 70S ribosomes and 50S subunits of Escherichia coli and proposed a theoretical model for adsorption when cooperative interaction occurs between ligands that are adsorbed on ribosomes. Analysis of the experimental binding isotherms leads to the following conclusions. A ribosome (or subunit) binds two C‐A‐C‐C‐A(3′NH)‐Phe molecules. The binding of C‐A‐C‐C‐A(3′NH)‐Phe to a ribosome (or subunit) is a cooperative process, characterized by a cooperativity coefficient τ=40±5 or more. The binding of C‐A‐C‐C‐A(3′NH)‐AcPhe at the donor site of the peptidyltransferase center (association binding constant 1.5×106 M−1) and the binding of puromycin at the acceptor site also occur cooperatively with a coefficient of 10–25, the association binding constant of puromycin at the acceptor site being (1–2)×104 M−1. The puromycin association binding constant at the donor site multiplied by the cooperativity coefficient of two interacting puromycin molecules adsorbed on a ribosome equals 100–200 M−1.

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