Determination of the complete amino acid sequence of protein S21 from Escherichia coli ribosomes.

Protein S21 was digested with trypsin before and after maleylation, with chymotrypsin, thermolysin and a glutamyl-specific protease. The resulting peptides were isolated and their amino acid composition determined. The amino acid sequences of selected peptides were determined either by the manual subtractive Edman method or by the dansyl-Edman procedure. Additional information was obtained from the automatic Edman degradation of the whole protein in a modified Sequenator. All these results combined yielded the sequence shown in Fig. 1. Protein S21 consists of 70 amino acids (Asp1,Asn2,Thr3,Ser2,Glu8,Pro3,Gly1,Ala9,Val6,Cys1,Ile1,Leu4,Tyr2,Phe3,His1,Lys9 and Arg14). It contains neither Met nor Trp. The molecular weight amounts to 8359. Clustering of basic amino acids is observed in five regions. We also include a prediction for regions with alpha-helices and with beta-sheets.