INITIATION-FACTORS IF1 AND IF2 SYNERGISTICALLY REMOVE PEPTIDYL-TRANSFER-RNAS WITH SHORT POLYPEPTIDES FROM THE P-SITE OF TRANSLATING ESCHERICHIA-COLI RIBOSOMES

A novel function of initiation factors IF1 and IF2 in Escherichia coli translation has been identified. It is shown that these factors effic iently catalyse dissociation of peptidyl-tRNAs with polypeptides of di fferent length from the P-site of E. coli ribosomes, and that the simu ltaneous presence of both factors is required for induction of drop-of f. The factor-induced drop-off occurs with both sense and stop codons in the A-site and competes with peptide elongation or termination. The efficiency with which IF1 and IF2 catalyse drop-off decreases with in creasing length of the nascent polypeptide, but is quite significant f or hepta-peptidyl-tRNAs, the longest polypeptide chains studied. In th e absence of IF1 and IF2 the rate of drop-off varies considerably for different peptidyl-tRNAs, and depends both on the length and sequence of the nascent peptide. Efficient factor-catalysed drop-off requires G TP but not GTP hydrolysis, as shown in experiments without guanine nuc leotides, with GDP or with the noncleavable analogue GMP-PNP. Simultan eous overexpression of IF1 and IF2 in vivo inhibits cell growth specif ically in some peptidyl-tRNA hydrolase deficient mutants, suggesting t hat initiation factor-catalysed drop-off of peptidyl-tRNA can occur on a significant scale in the bacterial cell. Consequences for the bacte rial physiology of this previously unknown function of IF1 and IF2 are discussed. (C) 1998 Academic Press.