Accumulation of free ribosomal proteins S1, L7, and L12 in Escherichia coli.

The total content of free ribosomal proteins in the cells of Escherichia coli was determined to study the nature of intracellular accumulation during growth. Labeled ribosomes and post-ribosomal supernatant were prepared from exponentially growing and stationary-phase cultures. The fraction of free ribosomal protein in the supernatant was estimated by resolving both the acidic and basic proteins separately with two different techniques of two-dimensional gel electrophoresis. Free ribosomal proteins in the cell sap were identified on the basis of coelectrophoresis with authentic ribosomal protein markers, molecular weights and amino acid composition. Among the acidic proteins, S1, L7, and L12 were identified and examined in detail. All three proteins accumulated to significant levels in these cultures. Stationary-phase cells contained 2-4 times more free S1, L7, and L12 than midlogarithmic phase cells. Moreover, free S1, L7, and L12 and ribosome-bound forms were stable during exponential and post-exponential growth of cultures. At this growth transition, non-ribosomal proteins in the supernatant and those associated with the ribosomes showed different characteristics of accumulation. The ratio of L12:L7 in the supernatant did not exhibit a remarkable shift during the growth cycle like the ratio of L12:L7 in ribosomes. In addition, free L12 in the supernatant was not acetylated, although there was a rapid acetylation in the cells.