[Synthesis of alkaline phosphatase in a stringent and a relaxed strain of Escherichia coli under amino acid and phosphate limitation]

Recent advances in cancer metabolism: a technological perspective

Cancer cells are highly dependent on metabolic pathways to sustain both their proliferation and adaption to harsh microenvironments. Thus, understanding the metabolic reprogramming that occurs in tumors can provide critical insights for the development of therapies targeting metabolism. In this review, we will discuss recent advancements in metabolomics and other multidisciplinary techniques that have led to the discovery of novel metabolic pathways and mechanisms in diverse cancer types.

Researchers now have access to a rapidly growing number of tools for probing the metabolic abnormalities associated with tumor growth. Unrestrained growth puts special demands on cancer cells, and scientists have known for nearly a century that tumor metabolism differs considerably from healthy tissue metabolism. Gina DeNicola and colleagues at the Moffitt Cancer Center and Research Institute, Tampa, USA, have reviewed the technological tools available for monitoring the molecules that power cell growth and survival. These include mass spectrometry, which can generate an extremely detailed census of cellular metabolites in a single experiment. The authors also highlight techniques that can help ‘trap’ short-lived or unstable chemical intermediates for analysis. Other chemical labeling and tracing techniques can illuminate activity of selected metabolic processes in living tumor cells or even in patients, findings that could reveal therapeutic vulnerabilities.

Guanosine 3',5'-bispyrophosphate (ppGpp) synthesis in cells of Escherichia coli starved for Pi

Cells of Escherichia coli which enter a phase of starvation for Pi induce the synthesis of the nucleotide guanosine 3',5'-bispyrophosphate (ppGpp). This induction is relA independent but depends on the spoT gene product. A mutant unable to produce ppGpp is impaired in the expression of two genes which belong to the pho regulon, a defect which is dependent on the product of spoT. We suggest that ppGpp is essential for the proper induction of the genes which belong to the pho regulon.

Synthesis of the stationary-phase sigma factor sigma s is positively regulated by ppGpp

Strains of Escherichia coli which lack detectable guanosine 3',5'-bispyrophosphate (ppGpp) display a pleiotropic phenotype that in some respects resembles that of rpoS (katF) mutants. This led us to examine whether ppGpp is a positive regulator of sigma s synthesis. sigma s is a stationary-phase-specific sigma factor that is encoded by the rpoS gene. We found that a ppGpp-deficient strain is defective in sigma s synthesis as cells enter stationary phase in a rich medium, as judged by immunoblots. Under more-defined conditions we found that the stimulation of sigma s synthesis following glucose, phosphate, or amino acid starvation of wild-type strains is greatly reduced in a strain lacking ppGpp. The failure of ppGpp-deficient strains to synthesize sigma s in response to these starvation regimens could indicate a general defect in gene expression rather than a specific dependence of rpoS expression on ppGpp. We therefore tested the effect of artificially elevated ppGpp levels on sigma s synthesis either with mutations that impair ppGpp decay or by gratuitously inducing ppGpp synthesis with a Ptac::relA fusion. In both instances, we observed enhanced sigma s synthesis. Apparently, ppGpp can activate sigma s synthesis under conditions of nutrient sufficiency as well as during entry into stationary phase. This finding suggests that changes in ppGpp levels function both as a signal of imminent stationary phase and as a signal of perturbations in steady-state growth.

Physiological studies on pBR 322 DNA amplification in an Escherichia coli relA strain

Amino acid limitation leads in E. coli relA cells which cannot synthesize guanosine tetraphosphate (ppGpp) under these conditions to an amplification of pBR 322 DNA. We previously proposed that ppGpp produced in E. coli relA+ cells subjected to amino acid limitation inhibits pBR 322 DNA replication (Hecker et al. 1983). In further experiments it was established that an E. coli relA strain shows plasmid amplification during amino acid limitation (arginine, threonine, leucine or histidine) only in the presence of sufficient concentrations of phosphate, ammonia and glucose. Plasmid amplification does not occur if ammonia or phosphate is depleted. We suggest that glucose, ammonia and phosphate are needed for nucleotide and deoxynucleotide synthesis as the essential prerequisite for plasmid amplification. The activity of beta-lactamase was determined as an indicator for the expression of plasmid-encoded genes. The enzyme activity remains on a low level during plasmid amplification because of arginine exhaustion. A remarkable increase in the activity of beta-lactamase was observed after resumption of growth of relA cells containing amplified plasmid DNA. The plasmid content decreased as the cells continued to grow. We found that plasmid amplification and expression of plasmid-localized genes are opposite reactions which do not occur at the same time.