Studies on the Role of an Escherichia coli 50 S Ribosomal Component in Polypeptide Chain Elongation

Abstract

The requirement for PI, a protein component of the 50 S ribosomal subunit of Escherichia coli shown previously to be essential for the interaction of the translation factors EF-T and EF-G with ribosomes, is minimized under certain conditions in partial reactions that can occur independently of chain elongation. Although very little EF-G binding occurs to PI-deficient ribosomes in the presence of GTP or GDP, significant amounts of EF-G can be bound with 5′-guanylylmethylenediphosphonate. EF-T-dependent GTP hydrolysis and EF-G-dependent binding of guanine nucleotides, as well as the associated EF-G-dependent GTPase activity carried out under stoichiometric conditions, were found to be much less dependent on PI in the presence of 20% methanol. These observations indicate that the catalytic sites of the PI-deficient-ribosomes are basically intact, and that PI plays an auxiliary role in enhancing the interaction of the elongation factors with the active sites of the ribosomes. End group analysis indicates that PI contains a minimum of two polypeptides with methionine and serine as NH2-terminal residues. Correspondence of PI to the ribosomal proteins reported by others is suggested on the basis of polyacrylamide gel pattern and amino acid composition.