Proteins crosslinked to messenger RNA by irradiating polyribosomes with ultraviolet light

A set of proteins crosslinked to L-cell mRNA by irradiating polyribosomes with 254 nm ultraviolet light has been identified. 35S-methionine-labeled crosslinked mRNA-protein complexes were isolated by chromatography on oligo(dT)-cellulose under conditions that prevented non-covalent binding of proteins to RNA and to the column. After enzymatic removal of the RNA the proteins were analyzed in sodium dodecylsulfate/polyacrylamide gels. Six proteins having molecular weights of 98,000, 78,000, 75,000, 68,000, 62,000, and 52,000 were crosslinked to mRNA whether intact polyribosomes or EDTA- released mRNA-protein complexes were irradiated. Digestions with specific RNAases and chromatography on oligo(dT)-cellulose were used to show that a protein of 78,000 daltons was the only one crosslinked to poly(A), and the other proteins were crosslinked to sequences other than poly(A). However, a 78,000 dalton protein was also crosslinked to a sequence other than poly(A).