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DksA and ppGpp are the central players in the stringent response and mediate a complete reprogramming of the transcriptome. A major component of the response is a reduction in ribosome synthesis, which is accomplished by the synergistic action of DksA and ppGpp bound to RNA polymerase (RNAP) inhibiting transcription of rRNAs. Here, we report the X-ray crystal structures of Escherichia coli RNAP in complex with DksA alone and with ppGpp. The structures show that DksA accesses the template strand at the active site and the downstream DNA binding site of RNAP simultaneously and reveal that binding of the allosteric effector ppGpp reshapes the RNAP-DksA complex. The structural data support a model for transcriptional inhibition in which ppGpp potentiates the destabilization of open complexes by DksA. This work establishes a structural basis for understanding the pleiotropic effects of DksA and ppGpp on transcriptional regulation in proteobacteria., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Cold-shocked cells of Escherichia coli can degrade intracellularly accumulated guanosine 5'-diphosphate 3'-diphosphate (ppGpp). The rate of ppGpp degradation is governed, as in whole cells, by the spoT gene; a rapid breakdown reaction is associated with the presence of the spoT+ allele and at least a five-fold slower decay occurs in spoT-minus mutants. The two degradation reactions in shocked cells display the following similarities: (i) the rates of degradation are equivalent to whole cell estimates, (ii) both require a full complement of activated amino acids, (iii) both are dependent upon supplements in the reaction mixture which stimulate the availability of energy-rich compounds and (iv) neither is inhibited by concentrations of ribosomal antibiotics which severely restrict protein synthesis. Apart from characteristic rate differences, decay of ppGpp in shocked cells derived from spoT-minus strains is discerned from spoT+ mediated decay in shocked cells by sensitivity to high concentrations of tetracycline and by manganese ion dependence.

relA and spoT are designations for two unlinked Escherichia coli genes whose products function in the synthesis and degradation of guanosine 3',5'-bispyrophosphate during the stringent regulatory response to amino acid deprivation. The RelA protein catalyzes an ATP:GTP 3'-pyrophosphoryl group transfer reaction, and the SpoT protein has a guanosine 3',5'-bispyrophosphate 3'-pyrophosphohydrolyase activity. Both genes have been sequenced recently; the relA gene produces an 84-kDa protein, and the spoT gene is deduced to encode a 79-kDa protein. We report here that the protein sequences of the relA and spoT genes are extensively interrelated.

This work describes an approach towards analyzing the regulatory effects of variation of guanosine 3',5'-bispyrophosphate (ppGpp) basal levels in Escherichia coli during steady state growth. A series of strains was derived by mutating the spoT gene (which encodes the major cellular ppGppase) so as to obtain systematic increments in ppGpp basal levels. These strains differ genetically at the spoT locus and, in some cases, also at the relA locus because of the severity of spoT mutant alleles. Measurements of ppGpp revealed a ten-fold range of basal levels during growth on minimal medium. The empirical relationship between ppGpp concentration and growth rate is a simple linear inverse correlation. Tandem rrnA ribosomal RNA promoters, present on a multicopy plasmid, are shown to be differentially regulated over this range of basal levels. The upstream P1 promoter activity shows an inverse exponential relation to ppGpp concentration whereas the downstream P2 promoter is only weakly affected. We conclude that there are systematic regulatory consequences associated with small changes in ppGpp basal levels during steady state growth that probably are part of a continuum with more dramatic effects observed during the stringent response to amino acid deprivation.

A cell-free system derived from E. coli is described in which mature-sized 16S and 23S ribosomal RNAs (rRNA) are synthesized at a high relative rate, comprising 17-25% of the total transcription. The addition of guanosine tetraphosphate (ppGpp) to this system results in up to a 5-fold selective inhibition of rRNA accumulation. This effect is exerted at the level of synthesis rather than degradation. It is concluded that ppGpp, which is produced in large amounts by E. coli during amino-acid deprivation, could mediate the decrease in rRNA synthesis that accompanies such deprivation. The expression of other genes has also been investigated. No selective reduction of transfer RNA synthesis by ppGpp is observed. The trp and lac operons are found to be stimulated at the transcriptional level by the presence of this nucleotide. It is hypothesized that ppGpp interacts with the RNA polymerase in such a manner as to alter the affinity of the enzyme for promoters in an operon-specific fashion.

The effects of guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), both produced by E. coli, were measured on the activities of several genes in a cell-free system. Gene activity is measured as gene-directed synthesis of biochemically competent protein or transfer RNA. Both ppGpp and pppGpp stimulated the activities of the ara, lac, and trp operons and inhibited the arg operon. Production of transfer-RNA(Tyr) was unaffected by moderate levels of either ppGpp or pppGpp and only slightly inhibited at higher levels of ppGpp. Since the cell-free reaction mixtures hydrolyze pppGpp to ppGpp, we performed similar studies with a hydrolysis-resistant analog of pppGpp, the beta-gamma methylenyl derivative (pcppGpp). In general, pcppGpp shows the same inhibitory potency as pppGpp for the arg operon, but lacks the stimulatory effects on the ara, lac, and trp operons. This result suggests that the stimulation of these gene activities is specific for ppGpp.Under similar conditions, pppGpp and ppGpp show a slight inhibitory effect on the messenger-directed synthesis of beta-galactosidase and no effect on the messenger-directed synthesis of MS2 viral-coat protein. These observations, together with the fact that in the same system these nucleotides affect coupled transcription and translation, lead us to surmise that the activities of pppGpp and ppGpp are exerted at the level of RNA polymerase activity.

The possible role of guanosine 5'-triphosphate,3'-diphosphate (pppGpp) in protein synthesis by Escherichia coli ribosomes and protein factors was examined. Although pppGpp could effectively substitute for GTP in reactions catalyzed by initiation factor 2 (ribosomal binding of fMet-tRNA and formation of N-formylmethionylpuromycin) and elongation factor T (ribosomal binding of Phe-tRNA and formation of dipeptidyl-tRNA), pppGpp poorly supported polyphenylalanine synthesis. The interaction of elongation factor G with pppGpp was, therefore, examined in detail. The nucleotide was found to be almost without activity in the translocation reaction, as measured by formation of N-acetylphenylalanyl-phenylalanylpuromycin. Nevertheless, the rate of the catalytic hydrolysis of pppGpp to guanosine 5'-diphosphate,3'-diphosphate by elongation factor G and ribosomes was about 30% of the rate of hydrolysis of GTP, a rate of hydrolysis that significantly exceeded the rate of translocation with GTP. Moreover, the rates of the fusidic acid-dependent, elongation factor G-dependent binding of pppGpp and ppGpp to ribosomes were about 75 to 85% the rates of GTP and GDP binding, respectively. We also found that dGTP could substitute for GTP in all reactions examined.

Guanosine tetraphosphate, recently discovered to mediate the regulatory relationship between protein synthesis and RNA accumulation in various bacteria, has been synthesized in vitro in large quantities and analyzed by natural-abundance (13)C nuclear magnetic resonance spectroscopy in order to confirm its structure and establish the positions of phosphate attachment. These studies have established its structure as guanosine 5'-diphosphate, 3'-diphosphate.