Coordinated expression of acetyl CoA synthetase and the ace operon enzymes in Escherichia coli in preparation for adaptation to acetate.

Successful adaptation of Escherichia coli to constant environmental challenges demands the operation of a wide range of regulatory control mechanisms, some of which are global, while others are specific. Here, we show that the ability of acetate-negative phenotype strains of E. coli devoid of acetate kinase (AK) and phosphotransacetylase (PTA) to assimilate acetate when challenged at the end of growth on acetogenic substrates is explicable by the co-expression of acetyl CoA-synthetase (AcCoA-S) and acetate permease (AP). Furthermore, mRNA transcript measurements for acs and aceA, together with the enzymatic activities of their corresponding enzymes, acetyl CoA synthetase (AcCoA-S) and isocitrate lyase (ICL), clearly demonstrate that the expression of the two enzymes is inextricably linked and triggered in response to growth rate threshold signal (0.4 h-1± 0.03: n4). Interestingly, further restriction of carbon supply to the level of starvation led to the repression of acs (AcCoA-S), ackA (AK) and pta (PTA). Further, we provide evidence that the reaction sequence catalysed by PTA, AK and AcCoA-S is not in operation at low growth rates and that the reaction catalysed by AcCoA-S is not merely an ATP-dissipating reaction but rather advantageous, as it elevates the available free energy (ΔG°) in central metabolism. Moreover, the transcriptomic data reinforce the view that the expression of PEP carboxykinase is essential in gluconeogenic phenotypes.

Longitudinal analysis reveals age-related changes in the T cell receptor repertoire of human T cell subsets.

A diverse T cell receptor (TCR) repertoire is essential for protection against a variety of pathogens, and TCR repertoire size is believed to decline with age. However, the precise size of human TCR repertoires, in both total and subsets of T cells, as well as their changes with age, are not fully characterized. We conducted a longitudinal analysis of the human blood TCRα and TCRß repertoire of CD4+ and CD8+ T cell subsets using a unique molecular identifier-based (UMI-based) RNA-seq method. Thorough analysis of 1.9 × 108 T cells yielded the lower estimate of TCR repertoire richness in an adult at 3.8 × 108. Alterations of the TCR repertoire with age were observed in all 4 subsets of T cells. The greatest reduction was observed in naive CD8+ T cells, while the greatest clonal expansion was in memory CD8+ T cells, and the highest increased retention of TCR sequences was in memory CD8+ T cells. Our results demonstrated that age-related TCR repertoire attrition is subset specific and more profound for CD8+ than CD4+ T cells, suggesting that aging has a more profound effect on cytotoxic as opposed to helper T cell functions. This may explain the increased susceptibility of older adults to novel infections.

SARS-CoV-2 B.1.1.7 (alpha) and B.1.351 (beta) variants induce pathogenic patterns in K18-hACE2 transgenic mice distinct from early strains.

SARS-CoV-2 variants of concern (VOC) B.1.1.7 (alpha) and B.1.351 (beta) show increased transmissibility and enhanced antibody neutralization resistance. Here we demonstrate in K18-hACE2 transgenic mice that B.1.1.7 and B.1.351 are 100-fold more lethal than the original SARS-CoV-2 bearing 614D. B.1.1.7 and B.1.351 cause more severe organ lesions in K18-hACE2 mice than early SARS-CoV-2 strains bearing 614D or 614G, with B.1.1.7 and B.1.351 infection resulting in distinct tissue-specific cytokine signatures, significant D-dimer depositions in vital organs and less pulmonary hypoxia signaling before death. However, K18-hACE2 mice with prior infection of early SARS-CoV-2 strains or intramuscular immunization of viral spike or receptor binding domain are resistant to the lethal reinfection of B.1.1.7 or B.1.351, despite having reduced neutralization titers against these VOC than early strains. Our results thus distinguish pathogenic patterns in K18-hACE2 mice caused by B.1.1.7 and B.1.351 infection from those induced by early SARS-CoV-2 strains, and help inform potential medical interventions for combating COVID-19.

Design of the Recombinant Influenza Neuraminidase Antigen Is Crucial for Its Biochemical Properties and Protective Efficacy.

Supplementing influenza vaccines with recombinant neuraminidase (rNA) antigens remains a promising approach for improving suboptimal vaccine efficacy. However, correlations among rNA designs, properties, and protection have not been systematically investigated. Here, we performed a comparative analysis of several rNAs produced by the baculovirus/insect cell system. The rNAs were designed with different tetramerization motifs and NA domains from a recent H1N1 vaccine strain (A/Brisbane/02/2018) and compared for enzymatic properties, antigenicity, stability, and protection in mice. We found that the enzymatic properties differ between rNAs containing the NA head domain versus the full ectodomain, the formation of higher-order rNA oligomers is tetramerization domain dependent, whereas the protective efficacy is more contingent on the combination of the tetramerization and NA domains. Following single-dose immunizations, an rNA possessing the full ectodomain and the tetramerization motif from the human vasodilator-stimulated phosphoprotein provided much better protection than an rNA with ∼10-fold more enzymatically active molecules that is comprised of the head domain and the same tetramerization motif. In contrast, these two rNA designs provided comparable protection when the tetramerization motif from the tetrabrachion protein was used instead. These findings demonstrate that individual rNAs should be thoroughly evaluated for vaccine development, as the heterologous domain combination can result in rNAs with similar key attributes that vastly differ in protection. IMPORTANCE For several decades, it has been proposed that influenza vaccines could be supplemented with recombinant neuraminidase (rNA) to improve efficacy. However, some key questions for manufacturing stable and immunogenic rNAs remain to be answered. We show here that the tetramerization motifs and NA domains included in the rNA construct design can have a profound impact on the biochemical, immunogenic, and protective properties. We also show that the single-dose immunization regimen is more informative for assessing the rNA immune response and protective efficacy, which is surprisingly more dependent on the specific combination of NA and tetramerization domains than common attributes for evaluating NA. Our findings may help to optimize the design of rNAs that can be used to improve or develop influenza vaccines.

Expression of the ace operon in Escherichia coli is triggered in response to growth rate-dependent flux-signal of ATP.

The signal that triggers the expression of the ace operon and, in turn, the transition of central metabolism's architecture from acetogenic to gluconeogenic in Escherichia coli remains elusive despite extensive research both in vivo and in vitro. Here, with the aid of flux analysis together with measurements of the enzymic activity of isocitrate lyase (ICL) and its aceA-messenger ribonucleuc acid (mRNA) transcripts, we provide credible evidence suggesting that the expression of the ace operon in E. coli is triggered in response to growth rate-dependent threshold flux-signal of adenosine triphosphate (ATP). Flux analysis revealed that the shortfall in ATP supply observed as the growth rate ($\mu $) diminishes from µmax to ≤ 0.43h-1 ($ \pm 0.02;n4)\ $is partially redressed by up-regulating flux through succinyl CoA synthetase. Unlike glycerol and glucose, pyruvate cannot feed directly into the two glycolytic ATP-generating reactions catalyzed by phosphoglycerokinase and pyruvate kinase. On the other hand, glycerol, which upon its conversion to D-glyceraldehyde, feeds into the phosphorylation and dephosphorylation parts of glycolysis including the substrate-level phosphorylation-ATP generating reactions, thus preventing ATP flux from dropping to the critical threshold signal required to trigger the acetate-diauxic switch until glycerol is fully consumed. The mRNA transcriptional patterns of key gluconeogenic enzymes, namely, ackA, acetate kinase; pta, phosphotransacetylase; acs, acetyl CoA synthetase and aceA, ICL, suggest that the pyruvate phenotype is better equipped than the glycerol phenotype for the switch from acetogenic to gluconeogenic metabolism.

Analyzing the Role of Gut Microbiota on the Onset of Autoimmune Diseases Using TNFΔARE Murine Model.

Very little is known about disease transmission via the gut microbiome. We hypothesized that certain inflammatory features could be transmitted via the gut microbiome and tested this hypothesis using an animal model of inflammatory diseases. Twelve-week-old healthy C57 Bl/6 and Germ-Free (GF) female and male mice were fecal matter transplanted (FMT) under anaerobic conditions with TNFΔARE-/+ donors exhibiting spontaneous Rheumatoid Arthritis (RA) and Inflammatory Bowel Disease (IBD) or with conventional healthy mice control donors. The gut microbiome analysis was performed using 16S rRNA sequencing amplification and bioinformatics analysis with the HIVE bioinformatics platform. Histology, immunohistochemistry, ELISA Multiplex analysis, and flow cytometry were conducted to confirm the inflammatory transmission status. We observed RA and IBD features transmitted in the GF mice cohort, with gut tissue disruption, cartilage alteration, elevated inflammatory mediators in the tissues, activation of CD4/CD8+ T cells, and colonization and transmission of the gut microbiome similar to the donors' profile. We did not observe a change or transmission when conventional healthy mice were FMT with TNFΔARE-/+ donors, suggesting that a healthy microbiome might withstand an unhealthy transplant. These findings show the potential involvement of the gut microbiome in inflammatory diseases. We identified a cluster of bacteria playing a role in this mechanism.

NGS Evaluation of Colorectal Cancer Reveals Interferon Gamma Dependent Expression of Immune Checkpoint Genes and Identification of Novel IFNγ Induced Genes.

To evaluate the expression of immune checkpoint genes, their concordance with expression of IFNγ, and to identify potential novel ICP related genes (ICPRG) in colorectal cancer (CRC), the biological connectivity of six well documented ("classical") ICPs (CTLA4, PD1, PDL1, Tim3, IDO1, and LAG3) with IFNγ and its co-expressed genes was examined by NGS in 79 CRC/healthy colon tissue pairs. Identification of novel IFNγ- induced molecules with potential ICP activity was also sought. In our study, the six classical ICPs were statistically upregulated and correlated with IFNγ, CD8A, CD8B, CD4, and 180 additional immunologically related genes in IFNγ positive (FPKM > 1) tumors. By ICP co-expression analysis, we also identified three IFNγ-induced genes [(IFNγ-inducible lysosomal thiol reductase (IFI30), guanylate binding protein1 (GBP1), and guanylate binding protein 4 (GBP4)] as potential novel ICPRGs. These three genes were upregulated in tumor compared to normal tissues in IFNγ positive tumors, co-expressed with CD8A and had relatively high abundance (average FPKM = 362, 51, and 25, respectively), compared to the abundance of the 5 well-defined ICPs (Tim3, LAG3, PDL1, CTLA4, PD1; average FPKM = 10, 9, 6, 6, and 2, respectively), although IDO1 is expressed at comparably high levels (FPKM = 39). We extended our evaluation by querying the TCGA database which revealed the commonality of IFNγ dependent expression of the three potential ICPRGs in 638 CRCs, 103 skin cutaneous melanomas (SKCM), 1105 breast cancers (BC), 184 esophageal cancers (ESC), 416 stomach cancers (STC), and 501 lung squamous carcinomas (LUSC). In terms of prognosis, based on Pathology Atlas data, correlation of GBP1 and GBP4, but not IFI30, with 5-year survival rate was favorable in CRC, BC, SKCM, and STC. Thus, further studies defining the role of IFI30, GBP1, and GBP4 in CRC are warranted.

Targeting specificity of APOBEC-based cytosine base editor in human iPSCs determined by whole genome sequencing.

DNA base editors have enabled genome editing without generating DNA double strand breaks. The applications of this technology have been reported in a variety of animal and plant systems, however, their editing specificity in human stem cells has not been studied by unbiased genome-wide analysis. Here we investigate the fidelity of cytidine deaminase-mediated base editing in human induced pluripotent stem cells (iPSCs) by whole genome sequencing after sustained or transient base editor expression. While base-edited iPSC clones without significant off-target modifications are identified, this study also reveals the potential of APOBEC-based base editors in inducing unintended point mutations outside of likely in silico-predicted CRISPR-Cas9 off-targets. The majority of the off-target mutations are C:G->T:A transitions or C:G->G:C transversions enriched for the APOBEC mutagenesis signature. These results demonstrate that cytosine base editor-mediated editing may result in unintended genetic modifications with distinct patterns from that of the conventional CRISPR-Cas nucleases.

Novel reference genes in colorectal cancer identify a distinct subset of high stage tumors and their associated histologically normal colonic tissues.

BACKGROUND: Reference genes are often interchangeably called housekeeping genes due to 1) the essential cellular functions their proteins provide and 2) their constitutive expression across a range of normal and pathophysiological conditions. However, given the proliferative drive of malignant cells, many reference genes such as beta-actin (ACTB) and glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) which play critical roles in cell membrane organization and glycolysis, may be dysregulated in tumors versus their corresponding normal controls METHODS: Because Next Generation Sequencing (NGS) technology has several advantages over hybridization-based technologies, such as independent detection and quantitation of transcription levels, greater sensitivity, and increased dynamic range, we evaluated colorectal cancers (CRC) and their histologically normal tissue counterparts by NGS to evaluate the expression of 21 "classical" reference genes used as normalization standards for PCR based methods. Seventy-nine paired tissue samples of CRC and their patient matched healthy colonic tissues were subjected to NGS analysis of their mRNAs. RESULTS: We affirmed that 17 out of 21 classical reference genes had upregulated expression in tumors compared to normal colonic epithelial tissue and dramatically so in some cases. Indeed, tumors were distinguished from normal controls in both unsupervised hierarchical clustering analyses (HCA) and principal component analyses (PCA). We then identified 42 novel potential reference genes with minimal coefficients of variation (CV) across 79 CRC tumor pairs. Though largely consistently expressed across tumors and normal control tissues, a subset of high stage tumors (HSTs) as well as some normal tissue samples (HSNs) located adjacent to these HSTs demonstrated dysregulated expression, thus identifying a subset of tumors with a potentially distinct and aggressive biological profile. CONCLUSION: While classical CRC reference genes were found to be differentially expressed between tumors and normal controls, novel reference genes, identified via NGS, were more consistently expressed across malignant and normal colonic tissues. Nonetheless, a subset of HST had profound dysregulation of such genes as did many of the histologically normal tissues adjacent to such HSTs, indicating that the HSTs so distinguished may have unique biological properties and that their histologically normal tissues likely harbor a small population of microscopically undetected but metabolically active tumors.

Trans-chalcone increases p53 activity via DNAJB1/HSP40 induction and CRM1 inhibition.

Naturally-occurring chalcones and synthetic chalcone analogues have been demonstrated to have many biological effects, including anti-inflammatory, anti-malarial, anti-fungal, and anti-oxidant/anti-cancerous activities. Compared to other chalcones, trans-chalcone exhibits superior inhibitory activity in cancer cell growth as shown via in vitro assays, and exerts anti-cancerous effects via the activation of the p53 tumor suppressor protein. Thus, characterization of the specific mechanisms, by which trans-chalcone activates p53, can aid development of new chemotherapeutic drugs that can be used individually or synergistically with other drugs. In this report, we found that trans-chalcone modulates many p53 target genes, HSP40 being the most induced gene in the RNA-Seq data using trans-chalcone-treated cells. CRM1 is also inhibited by trans-chalcone, resulting in the accumulation of p53 and other tumor suppressor proteins in the nucleus. Similar effects were seen using trans-chalcone derivatives. Overall, trans-chalcone could provide a strong foundation for the development of chalcone-based anti-cancer drugs.

Robust Sub-nanomolar Library Preparation for High Throughput Next Generation Sequencing.

BACKGROUND: Current library preparation protocols for Illumina HiSeq and MiSeq DNA sequencers require ≥2 nM initial library for subsequent loading of denatured cDNA onto flow cells. Such amounts are not always attainable from samples having a relatively low DNA or RNA input; or those for which a limited number of PCR amplification cycles is preferred (less PCR bias and/or more even coverage). A well-tested sub-nanomolar library preparation protocol for Illumina sequencers has however not been reported. The aim of this study is to provide a much needed working protocol for sub-nanomolar libraries to achieve outcomes as informative as those obtained with the higher library input (≥ 2 nM) recommended by Illumina's protocols. RESULTS: Extensive studies were conducted to validate a robust sub-nanomolar (initial library of 100 pM) protocol using PhiX DNA (as a control), genomic DNA (Bordetella bronchiseptica and microbial mock community B for 16S rRNA gene sequencing), messenger RNA, microRNA, and other small noncoding RNA samples. The utility of our protocol was further explored for PhiX library concentrations as low as 25 pM, which generated only slightly fewer than 50% of the reads achieved under the standard Illumina protocol starting with > 2 nM. CONCLUSIONS: A sub-nanomolar library preparation protocol (100 pM) could generate next generation sequencing (NGS) results as robust as the standard Illumina protocol. Following the sub-nanomolar protocol, libraries with initial concentrations as low as 25 pM could also be sequenced to yield satisfactory and reproducible sequencing results.

Serologic Evidence of Ebolavirus Infection in a Population With No History of Outbreaks in the Democratic Republic of the Congo.

Background: Previous studies suggest that cases of Ebola virus disease (EVD) may go unreported because they are asymptomatic or unrecognized, but evidence is limited by study designs and sample size. Methods: A large population-based survey was conducted (n = 3415) to assess animal exposures and behaviors associated with Ebolavirus antibody prevalence in rural Kasai Oriental province of the Democratic Republic of Congo (DRC). Fourteen villages were randomly selected and all healthy individuals ≥1 year of age were eligible. Results: Overall, 11% of subjects tested positive for Zaire Ebolavirus (EBOV) immunoglobulin G antibodies. Odds of seropositivity were higher for study participants older than 15 years of age and for males. Those residing in Kole (closer to the outbreak site) tested positive at a rate 1.6× higher than Lomela, with seropositivity peaking at a site located between Kole and Lomela. Multivariate analyses of behaviors and animal exposures showed that visits to the forest or hunting and exposure to rodents or duikers predicted a higher likelihood of EBOV seropositivity. Conclusions: These results provide serologic evidence of Ebolavirus exposure in a population residing in non-EBOV outbreak locations in the DRC and define statistically significant activities and animal exposures that associate with EBOV seropositivity.

Reducing acetate excretion from E. coli K-12 by over-expressing the small RNA SgrS.

When exposed to the nonmetabolized glucose derivative alpha methyl glucoside (αMG), both Escherichia coli K-12 (JM109 and MG1655) and E. coli B (BL21) respond by reducing the concentration of the mRNA of the ptsG gene which is responsible for the biosynthesis of the glucose transporter EIICB(glu). This occurs through the over-expression of the noncoding small RNA SgrS, which interacts specifically with the mRNA of the ptsG gene and prevents its translation. However, when these bacteria are exposed to a glucose concentration of 40 g/L, over-expression of SgrS is observed only in E. coli B (BL21). Unlike E. coli K-12 (JM109 and MG1655), which are affected by high glucose concentration and produce higher levels of acetate, E. coli B (BL21) is not affected. Based on this information, it was assumed that over-expression of SgrS enables E. coli B (BL21) to reduce its acetate excretion by controlling the glucose transport. When SgrS was over-expressed in both E. coli K-12 strains from a multicopy plasmid, it was possible to reduce their acetate excretion levels to those seen in E. coli B. This observation opens a new approach towards controlling bacterial metabolism through the use of noncoding RNA.

The role of Cra in regulating acetate excretion and osmotic tolerance in E. coli K-12 and E. coli B at high density growth.

BACKGROUND: E. coli B (BL21), unlike E.coli K-12 (JM109) is insensitive to glucose concentration and, therefore, grows faster and produces less acetate than E. coli K-12, especially when growing to high cell densities at high glucose concentration. By performing genomic analysis, it was demonstrated that the cause of this difference in sensitivity to the glucose concentration is the result of the differences in the central carbon metabolism activity. We hypothesized that the global transcription regulator Cra (FruR) is constitutively expressed in E. coli B and may be responsible for the different behaviour of the two strains. To investigate this possibility and better understand the function of Cra in the two strains, cra - negative E. coli B (BL21) and E. coli K-12 (JM109) were prepared and their growth behaviour and gene expression at high glucose were evaluated using microarray and real-time PCR. RESULTS: The deletion of the cra gene in E. coli B (BL21) minimally affected the growth and maximal acetate accumulation, while the deletion of the same gene in E.coli K-12 (JM109) caused the cells to stop growing as soon as acetate concentration reached 6.6 g/L and the media conductivity reached 21 mS/cm. ppsA (gluconeogenesis gene), aceBA (the glyoxylate shunt genes) and poxB (the acetate producing gene) were down-regulated in both strains, while acs (acetate uptake gene) was down-regulated only in E.coli B (BL21). These transcriptional differences had little effect on acetate and pyruvate production. Additionally, it was found that the lower growth of E. coli K-12 (JM109) strain was the result of transcription inhibition of the osmoprotectant producing bet operon (betABT). CONCLUSIONS: The transcriptional changes caused by the deletion of cra gene did not affect the activity of the central carbon metabolism, suggesting that Cra does not act alone; rather it interacts with other pleiotropic regulators to create a network of metabolic effects. An unexpected outcome of this work is the finding that cra deletion caused transcription inhibition of the bet operon in E. coli K-12 (JM109) but did not affect this operon transcription in E. coli B (BL21). This property, together with the insensitivity to high glucose concentrations, makes this the E. coli B (BL21) strain more resistant to environmental changes.

Acetate accumulation through alternative metabolic pathways in ackA (-) pta (-) poxB (-) triple mutant in E. coli B (BL21).

Individual deletions of acs and aceA genes in E. coli B (BL21) showed little difference in the metabolite accumulation patterns but deletion of the ackA gene alone or together with pta showed acetic acid gradually accumulated to 3.1 and 1.7 g/l, respectively, with a minimal extended lag in bacterial growth and a higher pyruvate formation. Single poxB deletion in E. coli B (BL21) or additional poxB deletion in the ackA-pta mutants did not change the acetate accumulation pattern. When the acetate production genes (ackA-pta-poxB) were deleted in E. coli B (BL21) acetate still accumulated. This may be an indication that perhaps acetate is not only a by-product of carbon metabolism; it is possible that acetate plays also a role in other cellular metabolite pathways. It is likely that there are alternative acetate production pathways.

Modified Escherichia coli B (BL21), a superior producer of plasmid DNA compared with Escherichia coli K (DH5alpha).

Plasmid DNA (pDNA) is an emerging experimental vaccine, produced in E. coli, initially targeted for viral diseases. Unlike traditional protein vaccines whose average dose is micrograms, the average dose of pDNA is on the scale of milligrams. Production yields are, therefore, important for the future development of this vaccine. The E. coli strains currently used for pDNA production, JM109 and DH5alpha, are both suitable for production of stable pDNA due to the deletion of recA and endA, however, these two E. coli K strains are sensitive to growth conditions such as high glucose concentration. On the other hand E. coli BL21 is less sensitive to growth conditions than E. coli JM109 or DH5alpha, this strain grows to higher densities and due to its active glyoxylate shunt and anaplerotic pathways is not sensitive to high glucose concentration. This strain is used for recombinant protein production but not for pDNA production because of its inability to produce stable pDNA. To adapt E. coli BL21 for stable pDNA production, the strain was mutated by deleting both recA and endA, and a proper growth and production strategy was developed. Production values, reaching 2 g/L were obtained using glucose as a carbon source. The produced plasmid, which was constructed for HIV clinical study, was found to have identical properties to the plasmid currently produced by E. coli DH5alpha.

Evaluating microarrays using a semiparametric approach: application to the central carbon metabolism of Escherichia coli BL21 and JM109.

Escherichia coli K (JM109) and E. coli B (BL21) are strains used routinely for recombinant protein production. These two strains grow and respond differently to environmental factors such as glucose and oxygen concentration. The differences have been attributed to differential expression of individual genes that constitute certain metabolic pathways that are part of the central carbon metabolism. By implementing a semiparametric algorithm, which is based on a density ratio model, it was possible to compare and quantify the expression patterns of groups of genes involved in several central carbon metabolic pathways. The groups comprising the glyoxylate shunt, TCA cycle, fatty acid, and gluconeogenesis and anaplerotic pathways were expressed differently between the two strains, whereas no differences were apparent for the groups comprising either glycolysis or the pentose phosphate pathway. These results further characterized differences between the two E. coli strains and illustrated the potency of the semiparametric algorithm.

Screen-less expanded bed column: new approach for the recovery and purification of a malaria transmission blocking vaccine candidate from Pichia pastoris.

An experimental malaria transmission blocking vaccine antigen, Pfs25H, expressed and secreted from Pichia pastoris was recovered and purified using a screenless expanded bed column equipped with a rotating fluid distribution system. This column was able to accommodate feed stock, containing 30% biomass, at a flow rate of 300-400 cm/h without affecting column stability. This capability is three times higher than the capability of the expanded bed column currently in use, which is equipped with a perforated plate fluid distribution system; this design could accommodate biomass concentrations of only up to 10%. The screen-less design did not affect the binding capacity, purification level or process yield and, therefore, shorten the process. Purified Pfs25H of 6.4 g were recovered from 37 l of Pichia pastoris culture in one step.

Impact of dissolved oxygen concentration on acetate accumulation and physiology of E. coli BL21, evaluating transcription levels of key genes at different dissolved oxygen conditions.

High density growth of Escherichia coli especially in large bioreactors may temporarily expose the cells to oxygen limitation as a result of a local inadequate oxygen supply or intermittently high concentrations of cells and nutrients. Although short, these periods can potentially alter bacterial metabolism, affecting both growth and recombinant proteins production capability, and thus lowering process productivity. When E. coli B (BL21), a lower acetate producing strain, was grown aerobically on high glucose, acetate accumulation was found to be inversely correlated to the dissolved oxygen (DO) levels, reaching 10 g/L at 1%, 4 g/L at 6%, and zero at 30% DO concentration at stationary growth phase. Time-course transcription analysis of several genes involved in glucose and acetate metabolism indicated that the enhanced acetate production at lower DO levels is the result of altered transcription of several key genes. These genes are: the acetate producing gene (poxB), the glyoxylate shunt gene (aceA), the acetate uptake gene (acs), the gluconeogensis and anaplerotic pathways genes, (pckA, ppsA, ppc, and sfcA), the TCA cycle gene (gltA), and the sigma factors 70 and S (rpoD and rpoS). It is suggested that the catabolic repressor/activator Cra is responsible for the bacterial response to different oxygen levels. Oxygen limitation seems to repress the constitutive expression of the glyoxylate shunt and gluconeognesis. In this work, the concept of transition state is proposed to describe the bacterial response to the lower DO concentration.