Runaway plasmid replication can be used to increase target gene dosage and thereby overproduce proteins within the bacterium Escherichia coli. However, the presence of excessive plasmid DNA often alters normal cell functions. High copy number plasmids with strong promoters place a severe metabolic burden on the cell, causing a decreased specific growth rate and changes in cell physiology. Induction of beta-lactamase synthesis from the tac promoter on plasmid pKN causes runaway plasmid replication and excretion of beta-lactamase. Runaway plasmid replication results from readthrough of tac promoter transcripts into the replication region of the plasmid. Both high plasmid copy numbers and a strong promoter (tac) are necessary to achieve the level of overproduction necessary for excretion of beta-lactamase, but high-level target protein synthesis is detrimental to the cell. A derivative of pKN which is more easily regulated was constructed by adding the lacI gene to the plasmid.
The biological treatment of volatile organic compounds (VOCs) and air toxics has received increased attention in recent years. Biotreatment of air‐borne contaminants offers an inexpensive alternative to conventional air treatment technologies such as carbon adsorption and incineration. Most biological air treatment technologies commercially available are fixed‐film systems that rely on growth of a biofilm layer on an inert organic support such as compost or peat (biofilters), or an inorganic support such as ceramic or plastic (biotrickling filters). If designed properly, these systems combine the advantages of high biomass concentration with high specific surface area for mass transfer.
At economically viable vapor residence times (1 to 1.5 minutes), biofilters can be used for treating vapor streams containing up to approximately 1500 μg/L of readily biodegradable compounds. Biotrickling filters may offer greater performance than biofilters at high contaminant loadings, possibly due to higher internal biomass concentrations. Both systems are best suited for treating vapor streams containing one or two major compounds. If designed properly, biofilters are especially well suited for treating streams that vary in concentration from minute to minute.
A new municipal wastewater treatment flowsheet was developed with the objectives of energy sustainability, and water and nutrient recovery. Energy is derived by shunting a large fraction of the organic carbon in the wastewater to an anaerobic digestion system. Aerobic and anaerobic membrane bioreactors play a key role in energy recovery. Phosphorus and nitrogen are removed from the wastewater and recovered through physical-chemical processes. Computer modeling and simulation results together with energy balance calculations, imply the new flowsheet will result in a dramatic reduction in energy usage at lower treatment plant capital costs in comparison to conventional methods.
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