Photo-induced affinity labeling of Escherichia coli ribosomes by chloramphenicol

In order to obtain more information about the binding site for chloramphenicol (D-threo diastereoisomer) on the bacterial ribosome, photo-affinity labeling experiments of this receptor have been performed with [3H]chloramphenicol itself. Control experiments show that this drug can be split photochemically by ultraviolet irradiation, whereas the ribosome is not modified structurally or functionally by such a treatment. When photolysis of a mixture of chloramphenicol and ribosomes is performed under critical conditions, some proteins like L1, L11, S3 and S4 are radiolabeled. L11, S3 and S4 are radiolabeled specifically as demonstrated by photo-incorporation experiments with isotopically diluted [3H]chloramphenicol or by comparison of the results obtained here with reversible experiments performed by the isotopic dilution method. When the D-erythro diastereoisomer of chloramphenicol is photo-incorporated into the bacterial ribosome, proteins are radiolabeled only in a non-specific way. These results show that this material could be used as an efficient scavenger. When finally D-threo [3H]chloramphenicol is photo-incorporated in the presence of a large amount of the D-erythro diastereoisomer, the radiolabeling pattern obtained for the proteins is quite different from that expected: while L11 is still labeled fairly extensively, L27 is the most radiolabeled protein found.