Cooperative effects in the peptidyltransferase center of Escherichia coli ribosomes.

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 tau = 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 X 10(6) 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) X 10(4) M-1. The puromycin association binding constant at the donor site multiplied by the cooperativity coefficient of two interacting puromycin molecules absorbed on a ribosome equals 100-200 M-1.