Direction of Chain Elongation in the Formation of Escherichia coli Ribosomal Protein

Ribosomal proteins were isolated from logarithmically growing Escherichia coli cells given [ 14 C]alanine for short periods. Surprisingly, the specific activity of alanine at the NH 2 -terminal end was higher than that of alanine released by carboxypeptidase A digestion of the ribosomal protein. To determine the direction of chain elongation more precisely, Escherichia coli cells were grown with [ 3 H]amino acids, and [ 14 C]amino acids were then given to the 3 H-labeled cells as a pulse. Radioactive 30S ribosomal proteins were isolated from these cells and subjected to carboxypeptidase digestion. The ratio of 14 C to 3 H of the released amino acids increased as the time for carboxypeptidase digestion progressed. These observations suggest that some of the ribosomal proteins may be synthesized from the carboxyl end to the amino end through a novel mechanism.