F'-episomes carrying the Salmonella typhimurium wild-type or attenuatordeleted histidine (his) operons were introduced into Escherichia coli strains containing relA or spoT single and double mutations known to affect guanosine 3'-diphosphate 5'-diphosphate (ppGpp) and guanosine 3'-triphosphate 5'-diphosphate (pppGpp) levels. Expression of the his operon and expression of the gene for 6-phosphogluconate dehydrogenase (gnd) were measured during balanced growth in amino acid-rich and minimal media. The data were consistent with the interpretation that ppGpp is a positive effector of his operon expression, whereas pppGpp is not an essential effector. The conclusion that his operon expression is maximally stimulated at a lower than maximum intracellular ppGpp concentration was further confirmed. Neither ppGpp nor pppGpp appeared to influence gnd gene expression. The metabolic regulation of the E. coli his operon was found to be similar to the ppGpp-mediated metabolic regulation of the S. typhimurium his operon.Guanosine 5'-diphosphate 3'-diphosphate (ppGpp) has been proposed as a positive effector of Salmonella typhimurium histidine (his) operon transcription both in vitro (21) and in vivo (21,26). The ppGpp-mediated effect on his operon expression in vivo has been termed "metabolic regulation" (21, 26). It has been suggested that ppGpp increases the mRNA initiation frequency from the his operon primary promoter in vivo by possibly acting as a direct effector of RNA polymerase (26).The conclusion that ppGpp acts as an in vivo effector has relied on a positive correlation between his operon expression and ppGpp levels for bacteria grown exponentially in amino acidrich medium versus minimal medium (26). The sampling showing this correlation was limited. Additional experiments suggesting ppGpp as an in vivo effector have been completed under conditions of unbalanced growth in strains which retained his operon attenuated control (hisO+) (21, 26). Therefore, although ppGpp clearly acts in vitro to stimulate his operon transcription (21), the evidence implicating ppGpp as an in vivo effector of his operon expression has been indirect.In this paper, we further examine the role of ppGpp as an in vivo positive effector of his operon expression in relA and spoT mutants of t Contribution no. Escherichia coli. The results were consistent with ppGpp as a positive effector of his operon expression during balanced growth. In contrast, guanosine 5'-triphosphate 3'-diphosphate (pppGpp) did not appear to play a necessary role in his operon metabolic regulation. MATERIALS AND METHODSBacterial strains and phage. The strains used or constructed in this study are given in Table 1. Auxotrophic markers were confirmed by growth tests. The presence or absence of the relA mutation in derivatives of strains WI, W2, W1l, and W21 was checked by a sensitivity test to the amino acid analogs ,Bchloroalanine, 2-thiazolealanine, and serine hydroxamate (J. A. Gallant, personal communication;21). In particular, strain W2 derivatives (relA+ spoT), which t...