Ribosomal proteins of Escherichia coli that stimulate stringent-factor-mediated pyrophosphoryl transfer in vitro.

Guanosine tetra- and pentaphosphate, (p)ppGpp, can be synthesized in vitro in a reaction containing only the enzyme (stringent factor), salts, and substrates (nonribosomal system). This reaction is greatly stimulated upon addition of methanol (methanol system) or by ribosomes, mRNA, and tRNA (ribosome system). Here we show that several ribosomal proteins alone stimulate the synthesis of (p)ppGpp in the presence of stringent factor (protein system). The optimal ionic conditions for the ribosome and protein systems are identical. The concentration of ribosomes or any stimulating ribosomal protein required for saturation of a given concentration of stringent factor is similar. Fifty of 54 ribosomal proteins were tested for stimulation in the protein system; 15 proteins showed high activity, seven of these from the 30S ribosomal subunit and eight from the 50S subunit. The physiological relevance of this finding is discussed.