Evidence for a repeated protein structure in the 30 S subunit of Escherichia coli ribosome

Protein S6 of Escherichia coli ribosome is an acidic protein on the 30 S subunit moiety and it exists in five forms differing only in the number of glutamic acid residues at the COOH terminus. I have determined the total number of copies of all five forms of S6 in the ribosome, using a wild type E. coli (MRE600) and a mutant strain (derivative of K12) that contains only one of these forms (Hitz, H., Schäfer, D., and Wittmann-Liebold, B. (1977) Eur. J. Biochem. 75,497-512). For this purpose, the ribosomes were labeled in vivo with 14C-, 3H-, or 35S-amino acids; bacteria were grown with a variety of carbon sources; and cells were harvested in midlogarithmic and stationary phases. The different forms of S6 were purified and a new procedure capable of electrophoretically separating the five forms of S6 from total ribosomal protein was developed. The results from these experiments show for the first time that there are two copies of protein S6 per ribosome. Thus, a repeat structure of a biochemically modified protein exists not only in the 50S subunit (protein L7/L12) as previously known, but also in the 30 S subunit of the ribosome.