50-S Ribosomal Proteins. Purification and Partial Characterization of Two Acidic Proteins, A1 and A2, Isolated from 50-S Ribosomes of Escherichia coli

Abstract

A method is presented to isolate in pure form two acidic proteins, indicated as A1- and A2-protein from 50-S ribosome of Escherichia coli. The advantage of this method is that it does not require prior isolation of subunits. The molecular weight of A1 and A2-protein was found by dodecylsulfate gel electrophoresis to be 11500 with a maximum uncertainty of 1500. Sedimentation equilibrium centrifugation in 5 M guanidine hydrochloride indicates a molecular weight in the range of 12000–13000. The amino acid composition of the two proteins is remarkable and indistinguishable. A-protein has a high content of alanine and does not contain tyrosine, tryptophan, histidine or cystenie. On gel-electrofocusing the isoelectric points of A1- and A2-protein are slightly different, namely pH 4.7 and pH 4.85. The amount of A-protein in actively dividing cells indicates that contrary to 30-S ribosomes, 50-S ribosomes have a restricted element of repeat. The number of copies of A-protein is found to be somewhat larger than two.