Polypeptide formation and polyribosomes in Escherichia coli treated with chloramphenicol

Abstract

In E. coli cultures maximally inhibited with chloramphenicol, formation of polypeptides still continued at a slow, constant rate for at least 90 min. The rate of leucine incorporation was reduced to 0.5%, but methionine was only reduced to 2%, suggesting that chains are normally initiated with methionine but are prematurely released at a short chain length. Consistent with this possibility was the distribution of the products on Sephadex columns: a range of peptides longer than 4 and shorter than 60 to 70 residues was seen. Less than 10% of the peptides broke down during a chase with cold amino acids, and during continuous labeling they accumulated progressively. On the average, one peptide was formed per ribosome every 5 min. Peptide synthesis in the presence of chloramphenicol was still dependent on ribosome translocation; it stopped in a mutant with an inactivated temperature sensitive elongation factor G. But even in the absence of translocation, new messenger ribonucleic acid (mRNA) chains were found joined to one or a few ribosomes. The chains had a size distribution comparable to that of mRNA from polyribosomes of growing cells. They were stabilized for an average time of about 5 min, but were more rapidly degraded after puromycin was added to the cells. This suggests that stabilization may be related to the average time spent by a ribosome on an mRNA chain, with or without polypeptide formation.