Stringent Control of Ribosomal lProtein Gi (coordinate regulation of rRNA and ribosomlal protein/gu

The regulation of the expression of ribo- somal protein genes was examined during partial inhibi- tion of tRNA aminoacylation in isogenic rel+ and rel- strains of E. coli B carrying a temperature-sensitive muta- tion in valyl-tRNA synthetase (EC 6.1.1.9) gene. Measure- ments of the differential synthesis rate of ribosomal pro- tein, ar (ribosomal protein synthesis rate/total protein synthesis rate), indicate that ribosomal protein synthesis is regulated directly or indirectly by availability of charged tRNA, and that the synthesis of ribosomal protein, like the synthesis of rRNA, is subject to the influence of the rel gene control system. In exponentially growing Escherichia coli the rate of ribosome production is dependent on the nutritional composition of the growth medium, and the synthesis rates of ribosomal com- ponents are balanced such that neither free rRNA nor free ribosomal proteins (r-proteins) accumulate in appreciable amounts (1). The synthesis of rRNA is regulated. One regula- tory mechanism is stringent control; during amino acid (or charged tRNA) deprivation in rel+ strains stable RNA ac- cumulation is severely (lepressed, while in rel- strains stable RNA accumulation continues at or near the pre-deprivation rate (2-4). Much less information is available regarding the regulation of the expression of r-protein genes (for reviews, see refs. 5-7). Maalde (5) has suggested that r-protein genes are expressed constitutively without any d(irect regulation and that the rate of r-protein synthesis may be passively regulated by the avail- ability of free RNA polymerase. However, it is also possible that, as in the case of rRNA, the synthesis of r-proteins is actively regulated, perhaps at the level of initiation of tran- scription. We have examined the role of the rel gene in the regulation of the synthesis of r-proteins; the rate of synthesis of r-pro- teins has been measured during partial deprivation of amino- acylated tRNA (AA-tRNA) in isogenic rel+ and rel- strains of E. coli B. The results demonstrate that the synthesis of r- protein is regulated by the availability of AA-tRNA, and that the product of the rel gene is involved in the regulation of r- protein synthesis as well as in the regulation of rRNA syn- thesis.