Coordinated clearance of periciliary liquid and mucus from airway surfaces.

Airway surface liquid is comprised of mucus and an underlying, watery periciliary liquid (PCL). In contrast to the well-described axial transport of mucus along airway surfaces via ciliary action, theoretical analyses predict that the PCL is nearly stationary. Conventional and confocal microscopy of fluorescent microspheres and photoactivated fluorescent dyes were used with well-differentiated human tracheobronchial epithelial cell cultures exhibiting spontaneous, radial mucociliary transport to study the movements of mucus and PCL. These studies showed that the entire PCL is transported at approximately the same rate as mucus, 39.2+/-4.7 and 39.8+/-4.2 micrometer/sec, respectively. Removing the mucus layer reduced PCL transport by > 80%, to 4.8+/-0.6 micrometer/sec, a value close to that predicted from theoretical analyses of the ciliary beat cycle. Hence, the rapid movement of PCL is dependent upon the transport of mucus. Mucus-dependent PCL transport was spatially uniform and exceeded the rate expected for pure frictional coupling with the overlying mucus layer; hence, ciliary mixing most likely accelerates the diffusion of momentum from mucus into the PCL. The cephalad movement of PCL along airway epithelial surfaces makes this mucus-driven transport an important component of salt and water physiology in the lung in health and disease.

Mobilization of broad host range plasmid from Pseudomonas putida to established biofilm of Bacillus azotoformans. I. Experiments.

A strain of Pseudomonas putida harboring plasmids RK2 and pDLB101 was exposed to a pure culture biofilm of Bacillus azotoformans grown in a rotating annular reactor under three different concentrations of the limiting nutrient, succinate. Experimental results demonstrated that the broad host range RSF1010 derivative pDLB101 was transferred to and expressed by B. azotoformans. At the lower concentrations, donor mediated plasmid transfer increased with increasing nutrient levels, but the highest nutrient concentration yielded the lowest rate of donor to recipient plasmid transfer. For transconjugant initiated transfer, the rate of transfer increased with increasing nutrient concentrations for all cases. At the lower nutrient concentrations, the frequency of plasmid transfer was higher between donors and recipients than between transconjugants and recipients. The reverse was true at the highest succinate concentration. The rates and frequencies of plasmid transfer by mobilization were compared to gene exchange by retrotransfer. The initial rate of retrotransfer was slower than mobilization, but then increased dramatically. Retrotransfer produced a plasmid transfer frequency more than an order of magnitude higher than simple mobilization.

Mobilization of broad host range plasmid from Pseudomonas putida to established biofilm of Bacillus azotoformans. II. Modeling.

A strain of Pseudomonas putida that harbors plasmids RK2 and pDLB101 was exposed to a pure culture biofilm of Bacillus azotoformans grown in a rotating annular reactor. Transfer of the RK2 mobilizable pDLB101 plasmid to B. azotoformans was monitored over a 4-day period. Experimental results demonstrated that the broad host range, RSF1010 derivative pDLB101 was transferred to and expressed by B. azotoformans. In the companion article to this work, the rate of plasmid transfer was quantified as a function of the limiting nutrient, succinate, and as a function of the mechanism of transfer. A biofilm process simulation program (AQUASIM) was modified to analyze resultant experimental data. Although the AQUASIM package was not designed to simulate or predict genetic events in biofilms, modification of the rate process dynamics allowed successful modeling of plasmid transfer. For the narrow range of substrate concentrations used in these experiments, nutrient level had only a slight effect on the rate and extent of plasmid transfer in biofilms. However, further simulations using AQUASIM revealed that under nutrient poor conditions, the number of transconjugants appearing in the biofilm was limited.

Continuous culture dynamics for aniline metabolism by Pseudomonas sp. CIT1.

Inhibition by toxic substrates enables multiple steady states to arise in biodegradation systems. This phenomenon was investigated for the continuous metabolism of aniline by Pseudomonas sp. CIT1. Differences of various metabolic parameters between the two growth regimes (uninhibited and inhibited) and the transient response to a step-up in dilution rate were determined. Regulatory mechanisms consistent with the experimental evidence are proposed. Aniline is the transcriptional inducer of a metabolic pathway that converts aniline to TCA cycle intermediates. The suite of enzymes is coordinately expressed from a single promoter. We followed the level of the pathway mRNA using a fragment containing the catechol 2,3 dioxygenase gene (andioxB) and monitored the pathway enzyme activity using catechol 2,3 dioxygenase (C23D). The inhibited regime resulted in a 60% lower growth yield, near constant levels of C23D monomer, but a 50% reduction in the specific activity of C23D, increased RNA synthesis rates (total and aniline pathway mRNA), and elevated RNA decay rates. Elucidation of regulatory mechanisms indicates that C23D is noncompetitively inhibited by aniline and subject to feedback inhibition by 2-hydroxymuconic semialdehyde (HMS). During uninhibited growth regime operation, metabolism of HMS is the rate-limiting step; in contrast, conversion of aniline to catechol limits growth in the inhibited regime.

Effects of medium carbon-to-nitrogen ratio on biofilm formation and plasmid stability.

Biofilm formation and plasmid segregational instability in biofilm cultures of Escherichia coli DH5alpha (pMJR1750) were investigated under different medium-carbon-to-nitrogen (C/N) ratios. At C/N ratios of 0.07 and 1, net accumulation of both biofilm plasmid-bearing and plasmid-free cells continued through the entire experiment without attaining any apparent steady state. At C/N ratios of 5 and 10, net biofilm cell accumulation for the two populations reached apparent steady states after 84 and 72 h, respectively. At C/N ratios of 0.07 and 1, polysaccharide production increased slowly and reached about 2g alginate equivalent/cm(2) by the end of both experiments. At a C/N ratio of 5, polysaccharide increase significantly after 84 h, reaching about 7 microg alginate equivalent/cm(2) prior to termination. At a C/N ratio of 10, polysaccharide increased significantly after 72 h and reached 21 microg alginate equivalent/cm(2) at 108 h. At C/N ratios of 0.07 and 1, protein production reached 6.5 and 4 microg/cm(2), respectively. At C/N ratios of 5 and 10, protein production increased slightly for the first 84 h and reached a maximum at 108 h, at 3 and 2 microg/cm(2), respectively, then decreased over the last 12 h of the experiment. Ratios of polysaccharide to protein increased with increasing C/N ratios. At C/N ratios of 0.07 and 1, the ratios between extracellular polysaccharide (EP) and protein were no more than 205 microg polysaccharide/microg protein, whereas those at C/N ratios of 5 and 10 increased to about 7 and 12 microg polysaccharide/microg protein, respectively.Probabilities of plasmid loss in the biofilm cultures increased with increasing C/N ratios. At C/N ratios of 0.07, 1, and 5, the probabilities of plasmid loss were 0.0013 +/- 0.011, 0.020 +/- 0.006 and 0.122 +/- 0.021, respectively. At a C/N ratio of 10, the probability of plasmid loss was significantly higher, reaching 0.38 +/- 0.125. The increase of probability of plasmid loss at higher C/N ratios results from competition between cell replication and extracellular polysaccharide production.

Stimulation of berberine secretion and growth in cell cultures of Thalictrum minus.

An endo-pectate lyase (PL; EC 4.2.2.2), originally cloned fiom the phytopathogenic bacterium Erwinia chrysanthemi EC16, was expressed in recA (-) E. coli strain DK1, purified to a single band by isoelectric focusing and used to induce berberine production in established plant suspension cultures of Thalictrum minus L. subsp. saxatile. Addition of 10(-9)M pectate lyase c (PLc) stimulated berberine production and enhanced secretion of the alkaloid into the medium. A lower concentration of PLc, 10(-11)M, stimulated a transient two-fold increase in cell growth rate relative to untreated cultures. Parallel changes in L-phenylalanine ammonia lyase (PAL; EC 4.3.1.5) activity with the rate of berberine synthesis and the inverse relationship between cell growth and berberine synthesis imply that berberine synthesis is stress-related in this cell line.

Plasmid retention and gene expression in suspended and biofilm cultures of recombinant Escherichia coli DH5alpha(pMJR1750).

Differences in plasmid retention and expression are studied in both suspended and biofilm cultures of Escherichia coli DH5alpha(PMJR1750). An alternative mathematical model is proposed which allows the determination of plasmid loss probability in both suspended batch and continuously fed biofilm cultures. In our experiments, the average probability of plasmid loss of E. coli DH5alpha(pMJR1750) is 0.0022 in batch culture in the absence of antibiotic selection pressure and inducer. Under the induction of 0.17 MM IPTG, the maximum growth rate of plasmid-bearing cells in suspended batch culture dropped from 0.45 h(-1) to 0.35 h(-1) and the beta-galactosidase concentration reached an experimental maximum of 0.32. pg/cell 4 hours after the initiation of induction. At both 0.34 and 0.51 mM IPTG, growth rates in batch cultures decreased to 0.16 h(-1), about 36% of that without IPTG, and the beta-galactosidase concentration reached an experimental maximum of 0.47 pg/cell 3 hours after induction.In biofilm cultures, both plasmid-bearing and plasmid-free cells in increase with time reaching a plateau after 96 hours n the absence of both the inducer and any antibiotic selection pressure. Average probability of plasmid loss for biofilm-bound E. coli DH5beta(pMJR1750) population was 0.017 without antibiotic selection. Once the inducer IPTG was added, the concentration of plasmid-bearing cells in biofilm dropped dramatically while plasmid-free cell numbers maintained unaffected. The beta-galactosidase concentration reached a maximum in all biofilm experiments 24 hours after induction; they were 0.08, 0.1, and 0.12 pg/cel under 0.17, 0.34, and 0.51 mM IPTG, respectively.

Analysis of productivity in lysis-deficient lambda expression systems.

The integrated state of lambda in the host chromosome in lysogeny can be combined with its extrachromosomal replication in the lytic state to achieve high cloned gene productivities. Our previous studies on lambda expression systems(21,22) have shown 100% segregational stability of the cloned gene in lysogeny and cloned gene product levels up to 15% of total cell protein in a mutant lytic state. However, the expression phase of systems based on Escherichia coli JM109 and JM105 showed partial lysis of the productive culture despite a mutation in the lysis gene S of the lambda vector resulting in extracellular release of the cloned gene product. In the current study, we have eliminated partial lysis in the expression phase of lambda systems and conducted a detailed comparative analysis of these systems in relation to maximization of cloned gene productivity. The elimination of partial cell lysis by using a nonpermissive strain Y1089 did not enhance product yields vs. earlier systems that exhibited partial lysis. The elimination of nonessential lambda protein production by construction of a new vector NP326 did not yield higher product yields presumably because of the small fraction of these proteins in the lytic state. Temperature induction of the lysogen Y1089(NM1070) resulted in higher product levels than direct infection of Y1089 by the phage vector at a high multiplicity. Using infection experiments, we found the promoter lacUV5 in the vector lambdaZEQS to yield threefold higher product levels than lac in NM1070, suggesting possible further enhancement of productivity with stronger promoters. The occurrence or absence of partial lysis in lambda systems could be used beneficially to achieve extracellular or intracellular product as desired. The large capacity of lambda vectors for insert DNA suggests potential applications in obtaining highly amplified levels of operons and multienzyme systems.

Characterization of the mutant lytic state in lambda expression systems.

The two propagative phases of bacteriophage lambda, lysogeny and lysis, can be used in concert to enhance productivity of recombinant expression systems. Lambda vectors carrying mutations to prevent both cell lysis and lambda DNA packaging in the lytic state have been shown to yield 100% stability of the product gene in lysogeny and to produce up to 15% of total cell protein as product beta-galactosidase in a mutant lytic state.(14) Despite these mutations, partial lysis of the culture was observed following induction of the cells from a lysogenic phase into the lytic state. To understand better the phage-host cell interactions and to investigate the possible cause(s) of lysis in these highly productive expression systems, we have made a detailed study of the suppressor-free system JM105(NM1070). We have found high levels of product (15% of total cell protein as beta-glactosidase) to be due chiefly to a high-copy number of lambda DNA in the mutant lytic state. There is partial lysis of the culture even in this suppressor-free system caused by a low-level natural suppression of the amber mutation in gene S of NM1070, resulting in accumulation of lambda endolysin. We have also monitored changes in cell growth and morphology upon induction of the lysogen. There is a slight increase in cell number that follows a linear relationship with time and a 25-fold increase in cell volume during recombinat protein production in the mutant lytic state.

Construction of a specialized-ribosome vector or cloned-gene expression in E. coli.

An expression system utilizing specialized ribosomes has been constructed with beta-galactosidase as the product. Ribosomes specific for lacZ mRNA are generated due to a mutation within the anti-Shine-Dalgarno region of a plasmidborne 16S rRNA gene that is complementary to a mutation within the ribosome-binding site of lacZ. Hence, a subpopulation of ribsomes specific for translation of the cloned gene mRNA is produced. Transcription of the lacZ gene is regulated by the tac promoter, while transcription of the mutated rrnB locus is controlled by the lambdaP(L) promoter. Batch experiments indicate that full induction of both operons (2 mM IPTG, 42 degrees C) leads to maximal beta-galactosidase activity per cell at levels 35% higher than that obtained using a wild-type ribosome expression system. Using a novel, site-directed mutagenesis technique, construction of the specialized ribosome vector is outlined, and the results of lacZ expression are presented as transcription of both the cloned-gene and the specialized-ribosome locus are induced.

Microfluorimetric analysis of spatial and temporal patterns of immobilized cell growth.

Pronounced spatial nonuniformities in cell density, physiology, and activity frequently arise within densely packed immobilized cell supports. For a more fundamental understanding of immobilized cell phenomena, we have developed high-resolution microfluorimetric procedures to analyze local variations in both immobilized cell loading and growth rate. Fluorescent staining of total cellular DNA provides a measure of local biomass density. Actively growing (DNA synthesizing) cells are marked by pulse-labeling newly synthesized DNA with the thymine analog, bromouracil. An immunofluorescent technique allows subsequent detection of spatial variations in DNA synthesis rates. These procedures enable the influence of mass-transfer limitations and other immobilization effects on cell distribution and activity to be readily quantified. We demonstrate this approach through analysis of the patterns of growth of Escherichia coli entrapped within Sr-alginate gel beads. The experimental techniques are potentially applicable to a variety of other aggregate cell systems.

Effect of chemically-induced, cloned-gene expression on protein synthesis in E. Coli.

Earlier experiments in our lab investigated the metabolic limitations of cloned-gene expression in bacterial cells (for over-production of beta-lactamase). These experiments showed that the steady-state concentration of ribosomal RNA decreased upon plasmid amplification while both the synthesis rate and steady-state beta-lactamase mRNA level increased significantly. This appeared to indicate substantial limitation exist within the transnational machinery of the bacterial cell at high copy numbers. To establish the generality of this phenomenon, the impact increasing protein expression from pa plasmid by chemically inducing a strong promoter while maintaining constant copy number has been investigated. A plasmid has been constructed which contains the lacZ gene under control of the tac promoter and contains the parB stability locus to maintain plasmid stability. Using this vector, beta-galactosidase expression in chemostat cultures operated at specific growth rates of 0.6 h(-1) was induced with IPTG such that enzyme activity was varied over a 460-fold range. When fully induced beta-galactosidase protein production represented 14 wt % of total cell protein. As transcription was induced, the synthesis rate of the beta-galactosidase mRNA increased 42-fold while the steady-state level of beta-galactosidase mRNA increased only fourfold. This indicates stability may play a larger role for beta-galactosidase expression with a strong promoter than seen with beta-lactamase production in the elevated copy number system. Furthermore, rRNA synthesis rates increased at high expression rates as seen in the copy number experiments. However, unlike the amplified-plasmid system, the steady-state levels of rRNA increased as well. Since the total protein levels closely followed the steady-state level of eRNA, transnational limitations are again suggested for the chemically induced transcription system.

Depression of protein synthetic capacity due to cloned-gene expression in E. coli.

Recombinant bacterial systems exhibit limited capacities for heterologous protein production. As seen with this and other systems, cloned-gene protein production reaches an upper limit despite further increases in gene dosage. A series of closely related plasmids which contain mutations affecting their copy number has been used to investigate the macromolecular impediments to enhanced recombinant protein production. Within a common host, HB101, the level of the ampicillin resistance gene, bla, was varied using five plasmids which differ solely in their replication machinery. Separate fermentations were conducted in which the plasmid copy number was varied from 0 to over 400 while the specific growth rate was fixed at 0.6 h(-1) for each chemostat cultivation. The effects of constitutive expression of the bla gene as copy number was elevated were then determined using pulse-labelling and RNA-DNA hybridizations. Specifically, the steady-state level, synthesis rate, and stability of beta-lactamase messenger RNA and ribosomal RNA were determined as a function of copy number. The results indicate that as copy number rises, both beta-lactamase mRNA synthesis rates and steady-state mRNA levels increase. Therefore, beta-lactamase production in these strains does not appear to be limited by the level of beta-lactamase mRNA. However, as the copy number was amplified, the stability of rRNA decreased to the point that steady-state levels of rRNA decreased. These data indicate that a limitation develops within the translational capacity of the cell at high levels of cloned-gene expression. The results suggest that strategies designed to enhance recombinant protein expression should include manipulation of translation as well as transcription.

Lambda vectors for stable cloned gene expression.

The bacteriophage lambda offers a unique opportunity concurrently to minimize segregational instability in recombinant systems by chromosomal integration of the cloned gene and to achieve high cloned gene expression during an abortive lytic phase. Lysis leads approximately to a 100-fold amplification of the cloned gene. Cell lysis in the lytic state is blocked by a specific mutation (Sam), allowing the cell to maintain its integrity, and lambda DNA packaging is blocked by other mutations (Wam, Eam) that keep cloned genes open to transcription. In the presence of these mutations, extremely high levels of cloned beta-galactosidase (more than 15% of total cell protein) have been obtained during abortive lysis from vectors found to be essentially 100% stable for over 75 generations in the lysogenic phase.

Transcription from plasmid genes, macromolecular stability, and cell-specific productivity in Escherichia coli carrying copy number mutant plasmids.

Experiments were performed to evaluate, qualitatively and quantitatively, the adaptation of Escherichia coli to plasmid maintenance and cloned gene expression. Experimental findings indicate that the metabolic response to low plasmid levels is an increase of the biosynthetic capacity of both transcription and translation. At high copy number levels the gene-specific transcription rate continues to increase but the stability of plasmid-derived mRNA drops sharply. Protein levels are maintained, but translation efficiency decreases. These results indicate that cellular biosynthetic capacity may not be limiting productivity in recombinant systems. If macromolecular stability is the bottleneck, then current efforts to increase gene expression that focus on enhancing synthesis rates will be ineffective.

Transient response simulations of recombinant microbial populations.

An asynchronous bacterial population has been approximated using a finite number of "computer" cells, each based on a complex single-cell model for Escherichia coli. This formulation correctly simulates the transient responses of protein and total cell mass synthesis rate to the sudden increase in the concentration of limiting energy source in the growth medium. Experimentally observed responses of rRNA and mRNA synthesis rates to growth rate shifts are qualitatively mirrored by the model. Simulation trends following those of a rel(-) mutant suggest that model modifications are needed to describe the dynamics of the stringent response. Simulations of the responses of recombinant populations to plasmid amplification or plasmid promoter induction also result in behavior similar to that determined experimentally. The calculated responses for recombinant populations subjected to constant promoter induction or cyclic induction-noninduction lead to the conclusion that inducible systems give greater productivity than those with fixed promoter strength. This formulation may be utilized as a basis for exploring other aspects of recombinant population dynamics.

Simulations of host-plasmid interactions in Escherichia coli: Copy number, promoter strength, and ribosome binding site strength effects on metabolic activity and plasmid gene expression.

A mechanistically detailed single-cell model E. coli B/r-A was adapted to simulate the effects of vector presence on cell metabolism. Competition for RNA polymerase between chromosome and plasmid DNA is explicitly included. Distribution of active ribosomes among chromosome- and plasmid-derived messenger RNA, another key facet of host-plasmid interactions, is also treated in detail. Simulations of recombinant cell growth rate and cloned-gene productivity as a function of relative plasmid number per cell agree closely with experimental results. Model prediction of the variation of cell cycle parameters C and D with plasmid number are roughly consistent with available data. Models of this class can be used to simulate changes in productivity resulting from specific alterations in the expression vector. The effects of changing cloned-gene promoter and ribosome binding strengths and of augmenting cell transcription or translation capacity have been studied using the recombinant cell model. Results suggest that cloned-gene expression is limited by cellular transcription capacity. These and other parametric studies, conveniently implemented using the computer cell, provide important guidance for future experiments directed at better understanding of host-plasmid interactions and at optimizing recombinant system productivity.

Mechanistically detailed model of cellular metabolism for glucose-limited growth of Escherichia coli B/r-A.

A structured mathematical model for cellular metabolism in Escherichia coli has been extended to encompass the mechanistic structure surrounding the kinetics and control of transcription and translation. The dependence of transcription on RNA polymerase and the mechanism of translation initiation have been explicitly included. This model correctly simulates cell growth, cell composition, and the timing of chromosome synthesis as a function of extracellular substrate concentration for glucose-limited balanced growth. Simulation results for the subpopulation of RNA polymerase engaged in transcription and for the distribution of this subpopulation among different promoter sites agree closely with experimental findings, as do calculated estimates of the active ribosomal fraction. In addition, the existence of an antitermination system for transcription of stable RNA operons is supported by model results. This model should provide a useful framework for investigating metabolic perturbations to E. coli, such as those resulting from insertion of extra-chromosomal vectors into the cells.

Studies of host-plasmid interactions in recombinant microorganisms.

Plasmid genes redirect some components of cellular metabolism into synthesis of plasmid gene products and additional plasmids. The stoichiometric and kinetic implications of these host-plasmid interactions have been investigated theoretically and experimentally. Using known pathway energetics, maximum theoretical yield factors based on ATP, glucose, and O2 have been estimated for recombinant Escherichia coli and compared with corresponding estimates for host cells alone, indicating major changes in carbon and energetic stoichiometry in recombinant cells in cases of high cloned gene expression. The influence of the number of plasmids in recombinant E. coli has been experimentally characterized using a series of pMB1 derivatives stably propagated at copy numbers from 12 to 408. Recombinant cell growth rate declines monotonically as plasmid content increases as does efficiency of plasmid gene expression. A detailed metabolically structured single-cell model for E. coli has successfully simulated these trends. Interrelationships among number of plasmids per cell, induction of expression of a plasmid gene, and recombinant population growth rate have been experimentally delineated for Saccharomyces cerevisiae containing plasmid pLGSD5 and derivatives in which the 2-micron origin of replication has been replaced by a cloned ARS1 sequence or its deletion fragments. The CEN4 centromere sequence has been included in some of these plasmids to provide more regular segregation. Specific growth rate of these recombinant yeasts exhibits a maximum as a function of plasmid content, an effect attributed to the interplay between beneficial effects of the plasmid in selective medium and parasitic effects on metabolism at larger plasmid content or with more plasmid gene expression activity. The yeast strains investigated exhibit substantial segregational instability that was characterized using a rapid-flow cytometry measurement based upon single-cell deletion of E. coli beta-galactosidase activity in recombinant cells.