HEAT AND IONIC LIMITATIONS DO NOT CHANGE THE INHIBITION PATTERN OF RIBOSOMAL PEPTIDYLTRANSFERASE BY AMINOHEXOSYL-CYTOSINE NUCLEOSIDE ANTIBIOTICS

In a cell-free system derived from Escherichia coli, we investigated t he inhibition of peptide bond formation by blasticidin S at 100 mM NH4 + and 5 degrees C or at 50 mM NH4+ and 25 degrees C. At both condition s, a transient phase of competitive inhibition is observed, followed b y a mixed noncompetitive phase. The two phases of inhibition are compa tible with a model in which a slow isomerization of the ribosome-drug complex occurs, as a result of ribosomal conformational changes. After this step, the mutually exclusive binding between acceptor substrate and antibiotic is converted to simultaneous binding. In comparison wit h a previous study carried out at 100 mM NH4+ and 25 degrees C, the pr esent results demonstrate that the ribosomal conformational changes in duced by blasticidin S can occur irrespectively of the reaction temper ature and the ionic conditions.