Identification of tyrosine residues that are susceptible to lactoperoxidase-catalyzed iodination on the surface of Escherichia coli 30S ribosomal subunit.

The detailed surface topography of the Escherichia coli 30S ribosomal subunit has been investigated, with iodination catalyzed by immobilized lactoperoxidase as the surface probe. Under mild conditions, only proteins S3, S7, S9, S18, and S21 were iodinated to a significant and reproducible extent. These proteins were isolated from the iodinated subunits, and in each case, the individual tyrosine residues that had reacted were identified by standard protein sequencing techniques. The targets of iodination that could be positively established were as follows: in protein S3 (232 amino acids), the tyrosines at positions 167 and 192; in S7 (153 amino acids), tyrosines 84 and 152; in S9 (128 amino acids), tyrosine 89; in S18 (74 amino acids), tyrosine 3 (tentative); in S21 (70 amino acids), tyrosines 37 and 70. The results represent part of a broader program to investigate ribosomal topography at the amino acid-nucleotide level.