Polysome Stability in Relaxed and Stringent Strains of Escherichia coli During Amino Acid Starvation

Abstract

The influence of amino acid starvation on polysome content was examined in relaxed and stringent strains of Escherichia coli which were isogenic for the RC locus. No difference was observed between the polysome profiles obtained from two different sets of stringent and relaxed strains starved for the same amino acid. In both relaxed and stringent strains, starvation for amino acids other than methionine resulted in only a slight breakdown of polysomes with a concomitant increase of 70 S ribosomes. However, starvation for methionine in both RC stringent and relaxed strains of E. coli resulted in a more extensive degradation of polysomes and accumulation of 70 S ribosomes. The 70 S ribosomes obtained as a result of methionine starvation were more sensitive to degradation to 50 and 30 S subunits in 10 −3 m Mg 2+ than 70 S monomers obtained either by degradation of polysomes with ribonuclease or by starvation of cells for amino acids other than methionine. The 70 S ribosomes from methionine starvation were similar (sensitivity to 10 −3 m Mg 2+ ) to 70 S ribosomes obtained from cells in which initiation of protein synthesis had been prevented by trimethoprim, an inhibitor of formylation. Since N -formyl-methionyl-transfer ribonucleic acid is required for initiation, the 70 S ribosomes obtained in both methionine-starved and trimethoprim-treated cells must result from association of 50 and 30 S subunits for reasons other than reinitiation. These results suggest that the level of ribonucleic acid synthesis does not influence the distribution of ribosomes in the polysome profile and vice versa.