Photoincorporation of Tetracycline into Rat‐Liver Ribosomes and Subunits

[3H]Tetracycline was covalently incorporated into rat liver ribosomes and isolated 40‐S and 60‐S subunits on irradiation at 254 nm. The antibiotic was almost exclusively incorporated into ribosomal proteins. At least some of these proteins are assumed to be involved in ribosomal function, since photoincorporated tetracycline was found to inhibit the activity of 40‐S and 60‐S subunits in the poly(U)‐directed protein‐synthesizing system as well as that of the 40‐S subunit in the poly(U)‐mediated [14C]Phe‐tRNA binding. The results from simultaneous one‐dimensional and two‐dimensional gel electrophoreses showed a small distribution of label among ribosomal proteins in 60‐S subunits and in 80‐S ribosomes, L10 being the most radioactive protein. As non‐acylated tRNA partly competed with this labeling, it is likely that tetracycline interaction with these proteins occurred at a functional site. L10 has already been found to interact with puromycin [Reboud, A. M., Dubost, S., Buisson, M. & Reboud, J. P. (1981) Biochemistry, 20, 5281–5288]. In the case of free 40‐S subunits the label distribution was wider among ribosomal proteins. No particular role has yet been found for the most labeled protein, S12, but protein S3a, which was also highly labeled, has already been reported to be involved in subunit function.