Overflow metabolism in Escherichia coli during steady-state growth: transcriptional regulation and effect of the redox ratio

Overflow metabolism in the form of aerobic acetate excretion by Escherichia coli is an important physiological characteristic of this common industrial microorganism. Although acetate formation occurs under conditions of high glucose consumption, the genetic mechanisms that trigger this phenomenon are not clearly understood. We report on the role of the NADH/NAD ratio (redox ratio) in overflow metabolism. We modulated the redox ratio in E. coli through the expression of Streptococcus pneumoniae (water-forming) NADH oxidase. Using steady-state chemostat cultures, we demonstrated a strong correlation between acetate formation and this redox ratio. We furthermore completed genome-wide transcription analyses of a control E. coli strain and an E. coli strain overexpressing NADH oxidase. The transcription results showed that in the control strain, several genes involved in the tricarboxylic acid (TCA) cycle and respiration were repressed as the glucose consumption rate increased. Moreover, the relative repression of these genes was alleviated by expression of NADH oxidase and the resulting reduced redox ratio. Analysis of a promoter binding site upstream of the genes which correlated with redox ratio revealed a degenerate sequence with strong homology with the binding site for ArcA. Deletion of arcA resulted in acetate reduction and increased the biomass yield due to the increased capacities of the TCA cycle and respiration. Acetate formation was completely eliminated by reducing the redox ratio through expression of NADH oxidase in the arcA mutant, even at a very high glucose consumption rate. The results provide a basis for studying new regulatory mechanisms prevalent at reduced NADH/NAD ratios, as well as for designing more efficient bioprocesses.

Nitrogen regulatory protein C-controlled genes of Escherichia coli: scavenging as a defense against nitrogen limitation

Nitrogen regulatory protein C (NtrC) of enteric bacteria activates transcription of genes/operons whose products minimize the slowing of growth under nitrogen-limiting conditions. To reveal the NtrC regulon of Escherichia coli we compared mRNA levels in a mutant strain that overexpresses NtrC-activated genes [glnL(Up)] to those in a strain with an ntrC (glnG) null allele by using DNA microarrays. Both strains could be grown under conditions of nitrogen excess. Thus, we could avoid differences in gene expression caused by slow growth or nitrogen limitation per se. Rearranging the spot images from microarrays in genome order allowed us to detect all of the operons known to be under NtrC control and facilitated detection of a number of new ones. Many of these operons encode transport systems for nitrogen-containing compounds, including compounds recycled during cell-wall synthesis, and hence scavenging appears to be a primary response to nitrogen limitation. In all, approximately 2% of the E. coli genome appears to be under NtrC control, although transcription of some operons depends on the nitrogen assimilation control protein, which serves as an adapter between NtrC and final sigma(70)-dependent promoters.

DNA microarray analysis of gene expression in response to physiological and genetic changes that affect tryptophan metabolism in Escherichia coli

We investigated the global changes in mRNA abundance in Escherichia coli elicited by various perturbations of tryptophan metabolism. To do so we printed DNA microarrays containing 95% of all annotated E. coli ORFs. We determined the expression profile that is predominantly dictated by the activity of the tryptophan repressor. Only three operons, trp, mtr, and aroH, exhibited appreciable expression changes consistent with this profile. The quantitative changes we observed in mRNA levels for the five genes of the trp operon were consistent within a factor of 2, with expectations based on established Trp protein levels. Several operons known to be regulated by the TyrR protein, aroF-tyrA, aroL, aroP, and aroG, were down-regulated on addition of tryptophan. TyrR can be activated by any one of the three aromatic amino acids. Only one operon, tnaAB, was significantly activated by the presence of tryptophan in the medium. We uncovered a plethora of likely indirect effects of changes in tryptophan metabolism on intracellular mRNA pools, most prominent of which was the sensitivity of arginine biosynthetic operons to tryptophan starvation.

Analysis of topoisomerase function in bacterial replication fork movement: use of DNA microarrays

We used DNA microarrays of the Escherichia coli genome to trace the progression of chromosomal replication forks in synchronized cells. We found that both DNA gyrase and topoisomerase IV (topo IV) promote replication fork progression. When both enzymes were inhibited, the replication fork stopped rapidly. The elongation rate with topo IV alone was 1/3 of normal. Genetic data confirmed and extended these results. Inactivation of gyrase alone caused a slow stop of replication. Topo IV activity was sufficient to prevent accumulation of (+) supercoils in plasmid DNA in vivo, suggesting that topo IV can promote replication by removing (+) supercoils in front of the chromosomal fork.

Mechanisms of separation of the complementary strands of DNA during replication

This article is a perspective on the separation of the complementary strands of DNA during replication. Given the challenges of DNA strand separation and its vital importance, it is not surprising that cells have developed many strategies for promoting unlinking. We summarize seven different factors that contribute to strand separation and chromosome segregation. These are: (1) supercoiling promotes unlinking by condensation of DNA; (2) unlinking takes place throughout a replicating domain by the complementary action of topoisomerases on precatenanes and supercoils; (3) topological domains isolate the events near the replication fork and permit the supercoiling-dependent condensation of partially replicated DNA; (4) type-II topoisomerases use ATP to actively unlink DNA past the equilibrium position; (5) the effective DNA concentration in vivo is less than the global DNA concentration; (6) mechanical forces help unlink chromosomes; and (7) site-specific recombination promotes unlinking at the termination of replication by resolving circular dimeric chromosomes.

Roles of topoisomerases in maintaining steady-state DNA supercoiling in Escherichia coli

DNA supercoiling is essential for bacterial cell survival. We demonstrated that DNA topoisomerase IV, acting in concert with topoisomerase I and gyrase, makes an important contribution to the steady-state level of supercoiling in Escherichia coli. Following inhibition of gyrase, topoisomerase IV alone relaxed plasmid DNA to a final supercoiling density (sigma) of -0.015 at an initial rate of 0.8 links min(-1). Topoisomerase I relaxed DNA at a faster rate, 5 links min(-1), but only to a sigma of -0.05. Inhibition of topoisomerase IV in wild-type cells increased supercoiling to approximately the same level as in a mutant lacking topoisomerase I activity (to sigma = -0.08). The role of topoisomerase IV was revealed by two functional assays. Removal of both topoisomerase I and topoisomerase IV caused the DNA to become hyper-negatively supercoiled (sigma = -0.09), greatly stimulating transcription from the supercoiling sensitive leu-500 promoter and increasing the number of supercoils trapped by lambda integrase site-specific recombination.

The mechanism of inhibition of topoisomerase IV by quinolone antibacterials

Topoisomerase IV (Topo IV) is a mediator of quinolone toxicity in bacteria. In this work, we demonstrate that norfloxacin, a model quinolone, converts Escherichia coli Topo IV into a poisonous adduct on DNA as opposed to inhibiting topoisomerase activity. Norfloxacin inhibition of Topo IV induces a slow decline in DNA synthesis that parallels cell death. Treatment of cells with a lethal concentration of the antibacterial did not block chromosome segregation, the phenotype of catalytic inhibition of Topo IV. Instead, norfloxacin causes DNA damage, as evidenced by the induction of the SOS pathway for DNA repair; the increase in susceptibility to the drug by mutations in genes for DNA repair pathways including recA, recB, and uvrD; and the efficient detergent-induced linearization of plasmid DNA in drug-treated cells. Wild-type and drug-resistant alleles of Topo IV are co-dominant, but we find that mutations in recA, seqA, or gyrB result in unconditional dominance of the sensitive allele, the characteristic of a poisoning mode of inhibition. These mutations either compromise chromosome integrity or force Topo IV to play a more active role in DNA unlinking in front of the replication fork. We interpret our results in terms of distinct but complementary roles of Topo IV and gyrase in DNA replication.

Topoisomerase IV, not gyrase, decatenates products of site-specific recombination in Escherichia coli

DNA replication and recombination generate intertwined DNA intermediates that must be decatenated for chromosome segregation to occur. We showed recently that topoisomerase IV (topo IV) is the only important decatenase of DNA replication intermediates in bacteria. Earlier results, however, indicated that DNA gyrase has the primary role in unlinking the catenated products of site-specific recombination. To address this discordance, we constructed a set of isogenic strains that enabled us to inhibit selectively with the quinolone norfloxacin topo IV, gyrase, both enzymes, or neither enzyme in vivo. We obtained identical results for the decatenation of the products of two different site-specific recombination enzymes, phage lambda integrase and transposon Tn3 resolvase. Norfloxacin blocked decatenation in wild-type strains, but had no effect in strains with drug-resistance mutations in both gyrase and topo IV. When topo IV alone was inhibited, decatenation was almost completely blocked. If gyrase alone were inhibited, most of the catenanes were unlinked. We showed that topo IV is the primary decatenase in vivo and that this function is dependent on the level of DNA supercoiling. We conclude that the role of gyrase in decatenation is to introduce negative supercoils into DNA, which makes better substrates for topo IV. We also discovered that topo IV has an unexpectedly strong DNA relaxation activity that, together with gyrase and topo I, is able to set the supercoiling levels in Escherichia coli.

Topoisomerase IV is a target of quinolones in Escherichia coli

We have demonstrated that, in Escherichia coli, quinolone antimicrobial agents target topoisomerase IV (topo IV). The inhibition of topo IV becomes apparent only when gyrase is mutated to quinolone resistance. In such mutants, these antibiotics caused accumulation of replication catenanes, which is diagnostic of a loss of topo IV activity. Mutant forms of topo IV provided an additional 10-fold resistance to quinolones and prevented drug-induced catenane accumulation. Drug inhibition of topo IV differs from that of gyrase. (i) Wild-type topo IV is not dominant over the resistant allele. (ii) Inhibition of topo IV leads to only a slow stop in replication. (iii) Inhibition of topo IV is primarily bacteriostatic. These differences may result from topo IV acting behind the replication fork, allowing for repair of drug-induced lesions. We suggest that this and a slightly higher intrinsic resistance of topo IV make it secondary to gyrase as a quinolone target. Our results imply that the quinolone binding pockets of gyrase and topo IV are similar and that substantial levels of drug resistance require mutations in both enzymes.