[3H-thymidine incorporation following isoleucine limitation in stringent and relaxed controlled strains of Escherichia coli].

Stringent and relaxed strains of E. coli subjected to isoleucine starvation were examined by follow-wing the incorporation of 3H-thymidine into chromosomal DNA. After valine treatment to trigger an isoleucine deprivation (p)ppGpp is synthesized in the stringent strain only. Remarkable differences in the morphology of the amino acid starved cells of the stringent and relaxed strains can be observed. Upon isoleucine limitation 3H-thymidine incorporation into DNA is reduced in both strains, but this inhibition is remarkably delayed in the relaxed strain. Our result show that the reduction of chromosomal DNA synthesis during amino acid limitation occurs also without ppGpp, but in the presence of ppGpp this process is accelerated.