Fluorescent labelling of Escherichia coli ribosomal sulfhydryl groups

The reactivity of the sulfhydryl groups of Escherichia coli ribosome has been investigated using a fluorescent label. Under denaturing conditions, all the --SH groups can be titrated. In the native form of the ribosome, six and less than one labels can be respectively conjugated to the 30-S and 50-S subparticles without loss of activity for poly(U)-dependent polyphenylalanine synthesis. The most reactive thiol groups belong to proteins S1, S12, S18, S21. The binding of mRNA plus tRNA to the 70-S ribosomes affects the spectroscopic properties of the labels showing that conformational changes are induced by these interactions. Furthermore, the treatment of these complexes by the labelling agent demonstrates that the --SH group belonging to protein S1 is partially protected whereas other thiol groups located on the 50-S subparticle become reactive.