Identification of Cysteine‐10 of Protein S18 as Part of the mRNA‐Binding Site of Escherichia coli Ribosomes by Affinity‐Labeling Studies with a Chemically Reactive A‐U‐G Analog

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Abstract
The reaction of a bromoacetamidophenyl derivative of the initiation codon A-U-G (*A-U-G [5''-{4-bromo-[2-14C]acetamido(phenyl)phospho}adenylyl-(3''-5'')uridylyl-(3''-5'')guanosine]) with tight couples of E. coli ribosomes leads to an exclusive crosslinking of label to protein S18. This crosslinking inhibits A-U-G-directed .**GRAPHIC**. binding into the puromycin-sensitive site of ribosomes and stimulates elongation-factor-dependent binding of .**GRAPHIC**. Apparently protein S18 is located at or near the aminoacyl-tRNA binding site of E. coli ribosomes. Peptide and amino acid analysis shows that the reaction between *A-U-G and ribosomes took place at cysteine-10 of protein S18. *A-U-G could not be crosslinked to ribosomal proteins of the temperature-sensitive E. coli strain 258ts, where arginine-11 of protein S18 is replaced by a cysteine residue.