Synthesis of NpnN’ (n = 3 or 4) in Vitro and in Vivo

This chapter describes the people present knowledge on the anabolism of the bis(5’-nucleosidyl) polyphosphates. Traces of pyrophosphate greatly inhibit Ap₄A production by aminoacyltRNA synhetases, probably through competitive inhibition of the ATP reaction with aminoacyl-adenylate. The effect of cognate tRNA on Ap₄A synthesis depends on the aminoacyl-tRNA synthetase. The case of dATP is particular since this nucleotide can react with the lysine to form a deoxyadenylate. The role of aminoacyl-tRNA synthetases in Ap₄N biosynthesis was probed by measuring the Ap₄N concentration in E. coli cells overproducing lysyl-, methionyl-, phenylalanyl-, or valyl-tRNA synthetases. Ap₄ N accumulates during heat shock whatever the concentration of the corresponding catabolic enzymes or the overproduction rate of any aminoacyl-tRNA synthetase. The characterization and mechanism of action of the enzymes which synthesize or degrade NpnN’ in living cells are of importance for the understanding of the regulation of the cellular concentration of these nucleotides.