Accumulation of 70 S Monoribosomes in Escherichia coli After Energy Source Shift-Down

Abstract

When Escherichia coli is shifted from glucose-minimal to succinate-minimal medium, a transient inhibition of protein synthesis and a time-dependent redistribution of ribosomes from polysomes to 70 S monosomes occurs. These processes are reversed by a shift-up with glucose. In a lysate made from a mixture of log-phase and down-shifted cells, the 70 S monosomes are derived solely from the down-shifted cells and are therefore not produced by polysome breakage during preparation. This conclusion is supported by the absence of nascent proteins from the 70 S peak. The monosomes are not dissociated by NaCl or by a crude ribosome dissociation factor, so they behave as “complexed” rather than “free” particles. When down-shifted cells are incubated with rifampin to block ribonucleic acid (RNA) synthesis, the 70 S monosomes disappear with a half-life of 15 min. When glucose is also added this half-life decreases to 3 min. The 70 S particles are stable in the presence of rifampin when chloramphenicol is added to block protein synthesis. We interpret these data to mean that the existence of the 70 S monosomes depends on the continued synthesis of messenger RNA and their conversion to free ribosomes (which dissociate under our conditions) is a result of their participation in protein synthesis. Finally, a significant fraction of the RNA labeled during a brief pulse of 3 H-uracil is found associated with the 70 S peak. These results are consistent with the hypothesis that the 70 S monosomes are initiation complexes of single ribosomes and messenger RNA, which do not initiate polypeptide synthesis during a shift-down.