DNA-dependent in vitro synthesis of Escherichia coli ribosomal protein S20 and isoleucyl-tRNA synthetase. Effect of guanosine-5'-diphosphate-3'-diphosphate.

Recent analysis by means of a direct chemical assay has shown that the numbers of aminoacyl-tRNA synthetase molecules per bacterial cell vary with the growth rate [ 1,2] . This type of control (‘metabolic regulation’) is responsible for the fact that under different rates of unrestricted growth the ratio of aminoacyl-tRNA synthetases relative to other components of the protein synthesis apparatus, especially ribosomes, remains more or less balanced [2] . Upon growth restriction, e.g., by amino acid starvation, however, aminoacyl-tRNA synthetases and ribosomes display a somewhat different response. In contrast to the well known ‘stringent response’ of ribosome synthesis under this condition, the amino acid activating enzymes seem to be only weakly affected by this control mechanism [3,4]. A suitable system for the in vitro investigation of the coordinate or non-coordinate regulation of synthesis of aminoacyl-tRNA synthetases and ribosomal components was provided by the isolation of a specialized transducing phage X carrying the structural genes for ribosomal protein S20 and for isoleucyl-tRNA synthetase [S] . DNA from this phage was used in this work to study how guanosine-5’-diphosphate-3’diphosphate (ppGpp) affects synthesis of isoleucyltRNA synthetase and of ribosomal protein S20 in a coupled transcription and translation system [6] since the different growth-rate-dependent basal levels of ppGpp have been implicated as a possible effector in the metabolic regulation of ribosome synthesis [7,8] during balanced growth. In addition, this nucleotide was demonstrated to drastically affect the in vitro synthesis of ribosomal RNA, ribosomal proteins and elongation factors [9-l l] .