Physical and Functional Homology between Ribosomal Protein S1 and Interference Factor i

The homology between the 30‐S ribosomal protein S1 and interference factor i has been investigated. The two proteins are indistinguishable by gel electrophoresis under various conditions. Similar peptides are obtained by cleavage with cyanogen bromide. After labelling with N‐ethylmaleimide, enzymatic digestion yields homologous labelled peptides. This homology is confirmed by immunological data. Characteristic activities of the two proteins have been compared. Both bind polyuridylic acid and stimulate polypeptide synthesis on this template by S1‐depleted ribosomes. Furthermore, S1 inhibits the translation of the coat protein cistron on MS2 RNA, while it has little effect on the overall translation of T4 mRNA, two characteristic properties of interference factor i. S1 and factor i are thus physically and functionally indistinguishable, which suggests that they have identical polypeptide chains. Implications are discussed in terms of function.