Metabolic changes in ribosomes of Escherichia coli during prolonged culture in different media

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Abstract
The metabolism of ribosomes during the exponential growth and post-exponential phase of E. coli cells was investigated. Incubation of E. coli cells in 2 rich media, L-broth and phosphate medium, up to stationary phase shows no drop in viability or any changes in ribosomes. The survival rate during prolonged culture of the post-stationary phase cells is a function of the incubation medium. The decline in viability is only slight in phosphate medium but very rapid in L-broth. So long as the viability is maintained, the level of ribosomes and the relative abundance of rRNA and ribosomal proteins in ribosomes of the post-stationary cultures are remarkably stable and are similar to exponentially growing cells. Post-stationary cultures undergoing a rapid drop in cell viability lose 95% of the original ribosomes. These cultures accumulate a large pool of 30S and 50S subunits and a few 70S monosomes, all of which show deficiency in the various ribosomal proteins. No differences in rRNA can be detected but the number and the relative stoichiometry of individual ribosomal proteins are drastically altered. Only 13 of the 53 proteins known in the E. coli ribosome appeared in the same relative amounts as in the ribosomes of the exponentially growing cells. Six proteins (S12, S21, L2, L16, L20, L34) are completely lost and all others under partial loss. An analysis of the number and relative abundance of ribosomal proteins in the whole cells, as opposed to isolated ribosomes, suggests that during the initial stages of the catabolism of ribosomes a crucial step is the formation of ribosomal-subunit-membrane complexes. The data emphasize the role of the constituents of ribosomes for the growth and the survival of E. coli cells. A model for the metabolism of ribosomes during the exponential growth and post-exponential phase of E. coli is presented.