In situ pH management for microbial culture in shake flasks and its application to increase plasmid yield

Shake flasks are widely used to culture microorganisms, but they do not allow for pH control without additional infrastructure. In the presence of a carbon source like glucose, culture pH typically decreases due to overflow metabolism and can limit the growth of microorganisms in shake flasks. In this study, we demonstrate the use of magnesium hydroxide-loaded pH managing hydrogels (m-pHmH) for in situ base release to counter the decrease in culture pH in shake flasks using Escherichia coli as a model organism, in both complex and mineral salts medium. Base release from m-pHmH is shown to increase with decreasing pH (22-fold increase in release rate from pH 8 to 5), thus providing feedback from culture pH. The addition of m-pHmH resulted in better pH maintenance and higher biomass yields of E. coli K12 in media containing glucose as a carbon source. The use of m-pHmH with additional buffer resulted in pH being maintained above 6.9 while pH decreases below 5 without m-pHmH. We demonstrate one application of such in situ pH management to increase the volumetric plasmid yield from E. coli in shake flask culture. In situ glucose release through a hydrogel to mimic fed-batch culture along with the addition of m-pHmH resulted in a 395 % increase in volumetric plasmid yield to 38 μg/ml in shake flask culture.     

Flexibility at Gly-194 is required for DNA cleavage and relaxation activity of Escherichia coli DNA topoisomerase I

The proposed mechanism of type IA DNA topoisomerase I includes conformational changes by the single enzyme polypeptide to allow binding of the G strand of the DNA substrate at the active site, and the opening or closing of the "gate" created on the G strand of DNA to the passing single or double DNA strand(s) through the cleaved G strand DNA. The shifting of an alpha helix upon G strand DNA binding has been observed from the comparison of the type IA DNA topoisomerase crystal structures. Site-directed mutagenesis of the strictly conserved Gly-194 at the N terminus of this alpha helix in Escherichia coli DNA topoisomerase I showed that flexibility around this glycine residue is required for DNA cleavage and relaxation activity and supports a functional role for this hinge region in the enzyme conformational change.     

Top-down N-terminal sequencing of Immunoglobulin subunits with electrospray ionization time of flight mass spectrometry

An N-terminal top-down sequencing approach was developed for IgG characterization, using high-resolution HPLC separation and collisionally activated dissociation (CAD) on a single-stage LCT Premier time of flight (TOF) mass spectrometer. Fragmentation of the IgG chains on the LCT Premier was optimized by varying the ion guide voltage values. Ion guide 1 voltage had the most significant effect on the fragmentation of the IgG chains. An ion guide 1 voltage value of 100 V was found to be optimum for the N-terminal fragmentation of IgG heavy and light chains, which are approximately 50 and 25 kDa, respectively. The most prominent ion series in this CAD experiment was the terminal b-ion series which allows N-terminal sequencing. Using this technique, we were able to confirm the sequence of up to seven N-terminal residues. Applications of this method for the identification of N-terminal pyroglutamic acid formation will be discussed. The method described could be used as a high-throughput method for the rapid N-terminal sequencing of IgG chains and for the detection of chemical modifications in the terminal residues.     

TimeView

TimeView is a MATLAB program that compares multiple temporal datasets from microarray experiments under two or more conditions, for example, temporal variation of cellular response upon exposure to different drugs. The current paucity of programs designed to efficiently compare and visualise gene expression profiles in such datasets led us to design TimeView, which also enhances data visualisation by plotting the expression profiles of a large number of genes on a single screen. AVAILABILITY: TimeView is available free of charge to all users at http://hugroup.cems.umn.edu/Research/Genomics/Timeview/timeview.htm. To use TimeView, users will require access to the commercial software MATLAB (version 6.5). A help document is available on the TimeView website.     

Elucidation of acid-induced unfolding and aggregation of human immunoglobulin IgG1 and IgG2 Fc

Understanding the underlying mechanisms of Fc aggregation is an important prerequisite for developing stable and efficacious antibody-based therapeutics. In our study, high resolution two-dimensional nuclear magnetic resonance (NMR) was employed to probe structural changes in the IgG1 Fc. A series of (1)H-(15)N heteronuclear single-quantum correlation NMR spectra were collected between pH 2.5 and 4.7 to assess whether unfolding of C(H)2 domains precedes that of C(H)3 domains. The same pH range was subsequently screened in Fc aggregation experiments that utilized molecules of IgG1 and IgG2 subclasses with varying levels of C(H)2 glycosylation. In addition, differential scanning calorimetry data were collected over a pH range of 3-7 to assess changes in C(H)2 and C(H)3 thermostability. As a result, compelling evidence was gathered that emphasizes the importance of C(H)2 stability in determining the rate and extent of Fc aggregation. In particular, we found that Fc domains of the IgG1 subclass have a lower propensity to aggregate compared with those of the IgG2 subclass. Our data for glycosylated, partially deglycosylated, and fully deglycosylated molecules further revealed the criticality of C(H)2 glycans in modulating Fc aggregation. These findings provide important insights into the stability of Fc-based therapeutics and promote better understanding of their acid-induced aggregation process.     

Analysis of miRNA regulation suggests an explanation for 'unexpected' increase in target protein levels

MicroRNA (miRNA) has been mostly associated with decrease in target protein expression levels. Recently, 'unexpected' observations of increase in target protein expression attributed to microRNA regulation have been reported. We formulate a comprehensive model for regulation by miRNA that includes both reversible mRNA-miRNA binding and selective return of RNA. We use this mathematical model incorporating multiple individual steps in the regulation process to study the simultaneous effects of these steps on the target protein level. We show that four dimensionless numbers obtained from 12 rate constants are sufficient to define the relative change in steady state target protein levels. We quantify the range of these numbers for which such pleiotropic increase in protein levels is possible, and interpret the experimental findings in the framework of our model such that the results are no longer unexpected. Finally, we show through stochastic simulation that the nature of the target protein distribution remains unchanged and the relative steady state noise levels are also completely defined by the values of these dimensionless numbers, irrespective of the individual reaction rate constants.