The Primary Structure of Ribosomal Protein S7 from E. Coli Strains K and B

Ribosomal proteins S7 from 30S subunits of Escherichia coli strains K and B differ extensively in their aminoacid compositions. The experimental details which led to the determination of the complete primary structures of proteins S7K and S7B are presented. Protein S7K consists of a single polypeptide chain of 177 aminoacids giving a calculated molecular weight of 19, 732, whereas protein S7B has 153 residues which amount to a molecular weight of 17,131. Aminoacid sequences were determined by a combination of automated Edman degradation of the intact proteins in a modified Beckman sequenator and sequencing of peptides obtained by digestion with trypsin. Staphylococcus aureus protease, thermolysin and pepsin, either by solid-phase Edman degradation or by dansyl-Edman degradation. Additional information about the primary structure was derived from peptides resulting from chemical cleavages of the protein by 2-(2-nitrophenyl-sulphenyl)-3-methyl 3' bromoindolenine at its tryptophanyl bonds and by cyanogen bromide at its methionyl bonds leading to large fragments. The mutational event occurring between S7B and S7K was characterized. Protein S7K contains an additional sequence of 24 aminoacids at its C-terminal end. The aminoacid sequence of both proteins S7K and S7B was compared to the published sequences of the other ribosomal proteins of Escherichia coli and predictions for the secondary structure of these proteins were made.