I 8 Number 231980 Nucleic Acids Research Proteins crossHnked to messenger RNA by irradiating polyribosomes with ultraviolet Kght

A set of proteins crossl inked to L-cell mRNA by Irradiat ing polyMbo-somes with 254 nm ul t ravio let l ight has been Ident i f ied. 35s-meth1on1ne-labeled crossHnked mRNA-protein complexes were Isolated by chromatography on oHgo(dT)-cel lulose 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 dodecylsui fate / polyacryl amide gels. Six proteins having molecular weights of 98,000, 78,000, 75,000, 68,000, 62,000, and 52,000 were crossHnked to mRNA whether Intact polyribosomes or EDTA-released mRNA-protein complexes were Irradiated. Digestions with specific RNAases and chromatography on ol1go(dT)-cellulose were used to show that a protein of 78,000 daitons was the only one crossHnked to poly(A), and the other proteins were crossHnked to sequences other than poly(A). However, a 78,000 dalton protein was also crosslinked to a sequence other than poly(A)