Accumulation of guanosine tetraphosphate induced by polymixin and gramicidin in Escherichia coli

The effects of two polypeptide antibiotics, polymixin B and gramicidin S, on the intracellular pool size and turnover of guanosine tetraphosphate (ppGpp) were analyzed in stringent (relA⁺) and relaxed (relA) strains of Escherichia coli. When either one of these two drugs was added to stringent bacteria cultures at a final concentration that blocked protein and RNA synthesis, ppGpp was found to accumulate. Under similar conditions of inhibition of macromolecular synthesis, ppGpp also appeared to accumulate in relaxed bacteria. Moreover, in either type of strain, no significant accumulation of guanosine pentaphosphate (pppGpp) could be detected upon drug treatment. It was, therefore, concluded that polymixin and gramicidin elicit ppGpp accumulation through a mechanism independent of the relA gene product and, consequently, quite distinct from the stringent control system triggered by amino acid starvation. Further experiments performed by using tetracycline as an inhibitor of ppGpp synthesis, showed that the increase in the level of this nucleotide induced by drug action was due, in fact, to a strong restriction of its degradation rate.