Production of mature bovine pancreatic ribonuclease in Escherichia coli

Expression of ribonuclease A and ribonuclease N1 in the filamentous fungus Neurospora crassa

In this study, we investigated the ability of the fungus Neurospora crassa to produce and secrete two ribonucleases: the heterologous bovine RNase A and the endogenous RNase N(1). A set of expression vectors was constructed, each consisting of an RNase A open reading frame under the control of a specific promoter and each with a specific terminator. N. crassa transformants were analyzed at the transcriptional and protein levels. Irrespective of the promoter used, all transformants showed an RNase A-specific transcript in northern hybridization, but transcriptional strengths differed significantly. The strongest transcription was detected in transformants under the control of the cfp promoter. Western blot analysis and ELISA assays of selected transformants showed an effective secretion up to 356 ng/mL of recombinant RNase A protein. However, the highest ribonuclease activity could be detected in transformants carrying the endogenous RNase N(1) under the control of the ccg1 promoter. Expression and secretion of RNase N(1) thus represent an alternative to recombinant expression of RNase A protein. In conclusion, we have created a viable expression system for expression of homologous and heterologous proteins in N. crassa.

An efficient method of selectable marker gene excision by Xer recombination for gene replacement in bacterial chromosomes

A simple, effective method of unlabeled, stable gene insertion into bacterial chromosomes has been developed. This utilizes an insertion cassette consisting of an antibiotic resistance gene flanked by dif sites and regions homologous to the chromosomal target locus. dif is the recognition sequence for the native Xer site-specific recombinases responsible for chromosome and plasmid dimer resolution: XerC/XerD in Escherichia coli and RipX/CodV in Bacillus subtilis. Following integration of the insertion cassette into the chromosomal target locus by homologous recombination, these recombinases act to resolve the two directly repeated dif sites to a single site, thus excising the antibiotic resistance gene. Previous approaches have required the inclusion of exogenous site-specific recombinases or transposases in trans; our strategy demonstrates that this is unnecessary, since an effective recombination system is already present in bacteria. The high recombination frequency makes the inclusion of a counter-selectable marker gene unnecessary.

A modified Escherichia coli protein production strain expressing staphylococcal nuclease, capable of auto-hydrolysing host nucleic acid

The large-scale production of recombinant biotherapeutics, particularly recombinant proteins, provides significant process and regulatory challenges to the biotechnology industry in order to meet the regulatory agencies stringent requirements in a cost-effective manner. Host cell derived nucleic acid causes problems from both a process and a regulatory perspective, as high molecular weight chromosomal DNA is responsible both for the viscosity of cell lysates, and it is a source of heterologous DNA sequences whose inclusion in the final product must be prevented. We have constructed a modified Escherichia coli JM107 expression host (JMN), containing a staphylococcal nuclease expression cassette, integrated into the host chromosome at the dif locus. The nuclease is expressed as a fusion to the ompA signal peptide, and is translocated to the periplasm of the cell, protecting the cytoplasmic nucleic acid from any toxic activity. The nuclease is released during cell lysis, where it subsequently acts to hydrolyse host nucleic acid present in the lysate. Results with this strain show that sufficient levels of nuclease activity are produced to completely auto-hydrolyse the host's chromosomal DNA to a size non-visible on 1% agarose gel, generating a markedly lower lysate viscosity. This provides a suitable methodology to remove heterologous DNA sequences early in the product stream and decrease lysate viscosity, improving the efficiency of downstream processing and product yield, whilst avoiding the addition of exogenous nuclease and its prohibitive costs at large-scale.

Purification of essentially RNA free plasmid DNA using a modified Escherichia coli host strain expressing ribonuclease A

Regulatory agencies have stringent requirements for the large-scale production of biotherapeutics. One of the difficulties associated with the manufacture of plasmid DNA for gene therapy is the removal of the host cell-related impurity RNA following cell lysis. We have constructed a modified Escherichia coli JM107 plasmid host (JMRNaseA), containing a bovine pancreatic ribonuclease (RNaseA) expression cassette, integrated into the host chromosome at the dif locus. The expressed RNaseA is translocated to the periplasm of the cell, and is released during primary plasmid extraction by alkaline lysis. The RNaseA protein is stable throughout incubation at high pH ( approximately 12-12.5), and subsequently acts to hydrolyse host cell RNA present in the neutralised solution following alkaline lysis. Results with this strain harbouring pUC18, and a 2.4 kb pUC18DeltalacO, show that sufficient levels of ribonuclease (RNase) activity are produced to hydrolyse the bulk of the host RNA. This provides a suitable methodology for the removal of RNA, whilst avoiding the addition of exogenous animal sourced RNase and its associated regulatory requirements.

Drug metabolism by Escherichia coli expressing human cytochromes P450

The broad substrate specificity of the cytochrome P450 (P450) enzyme superfamily of heme-thiolate proteins lends itself to diverse environmental and pharmaceutical applications. Until recently, the primary drawback in using living bacteria to catalyze mammalian P450-mediated reactions has been the paucity of electron transport from NADPH to P450 via endogenous flavoproteins. We report the functional expression in Escherichia coli of bicistronic constructs consisting of a human microsomal P450 enzyme encoded by the first cistron and the auxiliary protein NADPH-P450 reductase by the second. Expression levels of P450s ranged from 35 nmol per liter culture to 350 nmol per liter culture, with expression of NADPH-P450 reductase typically ranging from 50% to 100% of that of P450. Transformed bacteria metabolized a number of typical P450 substrates at levels comparable to isolated bacterial membranes fortified with an NADPH-generating system. These rates compare favorably with those obtained using human liver microsomes as well as those of reconstituted in vitro systems composed of purified proteins, lipids, and cofactors.

Strategies for achieving high-level expression of genes in Escherichia coli

Progress in our understanding of several biological processes promises to broaden the usefulness of Escherichia coli as a tool for gene expression. There is an expanding choice of tightly regulated prokaryotic promoters suitable for achieving high-level gene expression. New host strains facilitate the formation of disulfide bonds in the reducing environment of the cytoplasm and offer higher protein yields by minimizing proteolytic degradation. Insights into the process of protein translocation across the bacterial membranes may eventually make it possible to achieve robust secretion of specific proteins into the culture medium. Studies involving molecular chaperones have shown that in specific cases, chaperones can be very effective for improved protein folding, solubility, and membrane transport. Negative results derived from such studies are also instructive in formulating different strategies. The remarkable increase in the availability of fusion partners offers a wide range of tools for improved protein folding, solubility, protection from proteases, yield, and secretion into the culture medium, as well as for detection and purification of recombinant proteins. Codon usage is known to present a potential impediment to high-level gene expression in E. coli. Although we still do not understand all the rules governing this phenomenon, it is apparent that "rare" codons, depending on their frequency and context, can have an adverse effect on protein levels. Usually, this problem can be alleviated by modification of the relevant codons or by coexpression of the cognate tRNA genes. Finally, the elucidation of specific determinants of protein degradation, a plethora of protease-deficient host strains, and methods to stabilize proteins afford new strategies to minimize proteolytic susceptibility of recombinant proteins in E. coli.