Assembly of enhancers, promoters, and splice signals to control expression of transferred genes

In vitro recombination cloning of entire cDNA libraries in Arabidopsis thaliana and its application to the yeast two-hybrid system

In the postgenomic era many experiments rely on the availability of transcript sequence for cloning. As these clones usually originate from cDNA libraries, the quality of these libraries is crucial. If a good library is generated it is desirable to use a versatile cloning system suitable for many different kinds of applications. The cloning systems based on in vitro recombination proves fitting for this task. However, the use of this method for shuttling entire cDNA libraries between different vectors has not yet been studied in great detail. Here we describe the construction of four cDNA libraries from different tissues of Arabidopsis thaliana, the shuttling of the libraries into expression vectors, and evaluation of this method as well as its suitability for downstream applications. Libraries were constructed from seedlings, hormone treated seedlings, flowers, developing seeds and primary leaves in the "entry vector" of the Gateway cloning system. After initial characterization of the libraries, they were shuttled into an expression vector (a yeast two-hybrid prey vector). To monitor for a size bias generally assumed to be inherent to in vitro recombination methods, the libraries were characterized before and after the transfer into the expression vector. However no significant difference could be detected. The functionality of the in vitro recombination system for the shuttling of entire libraries was then further tested by protein-protein interaction screens. The results of the library characterization and of the yeast two-hybrid screens and their implications for large-scale proteomic approaches are discussed.

Components of vectors for gene transfer and expression in mammalian cells

Progress in diverse scientific fields has been realized partly by the continued refinement of mammalian gene expression vectors. A growing understanding of biological processes now allows the design of vector components to meet specific objectives. Thus, gene expression in a tissue-selective or ubiquitous manner may be accomplished by selecting appropriate promoter/enhancer elements; stabilization of labile mRNAs may be effected through removal of 3' untranslated regions or fusion to heterologous stabilizing sequences; protein targeting to selected tissues or different organelles is carried out using specific signal sequences; fusion moieties effect the detection, enhanced yield, surface expression, prolongation of half-life, and facile purification of recombinant proteins; and careful tailoring of the codon content of heterologous genes enhances protein production from poorly translated transcripts. The use of viral as well as nonviral genetic elements in vectors allows the stable replication of episomal elements without the need for chromosomal integration. The development of baculovirus vectors for both transient and stable gene expression in mammalian cells has expanded the utility of such vectors for a broad range of cell types. Internal ribosome entry sites are now widely used in many applications that require coexpression of different genes. Progress in gene targeting techniques is likely to transform gene expression and amplification in mammalian cells into a considerably less labor-intensive operation. Future progress in the elucidation of eukaryotic protein degradation pathways holds promise for developing methods to minimize proteolysis of specific recombinant proteins in mammalian cells and tissues.

Insertion of metal-responsive elements increases the inducibility of the mouse mammary tumor virus promoter

The promoter of the mouse mammary tumor virus (MMTV) is commonly used in inducible mammalian expression vectors. We have modified this promoter by inserting metal-responsive elements (MRE) at different sites. Surprisingly, MMTV promoters containing MRE 5' with respect to the glucocorticoid responsive elements (GRE), were not induced by metal ions. However, in the simultaneous presence of dexamethasone and metal ions, the modified promoters displayed a significantly increased transcriptional strength [up to sevenfold more than wild-type (wt) MMTV] in transiently transfected cells and permanent cell lines. This increment in the inducible expression was achieved without a concomitant increase in the basal level of expression. These modified MMTV promoters will offer the opportunity to develop tightly regulated inducible mammalian expression systems that are significantly more potent than the ones based on the wt MMTV promoter.

Genetically engineering mammalian cell lines for increased viability and productivity

The generation of new host cell lines for the production of foreign proteins can be achieved by cell engineering. This approach can be used to enhance the cell's ability to produce proteins that are properly processed and secreted at elevated levels and consequently can increase the overall productivity of an expression system. One potential target for cell engineering is the modification of the cell's protein folding capacity. The appropriate folding, assembly, localization and secretion of newly synthesized proteins is dependent upon the action of a group of proteins known as molecular chaperones. Improving the host cell's chaperoning capacity might increase the yield of properly folded recombinant proteins by preventing the formation of insoluble aggregates. Another potentially beneficial cell engineering goal is the inhibition of physiological cell death. The productivity of genetically engineered cells is dependent upon the maintenance of high levels of cell viability throughout the bioprocess period. Fluctuations in a cell's environment can trigger a deliberate form of cell death known as apoptosis. The proteins that mediate this self-destruction are currently being characterized. Regulating the expression of these death genes by cellular engineering could limit the loss of productivity that results from the physiological death of the recombinant cell line.

Constitutive expression of the urokinase plasminogen activator gene in murine RAW264 macrophages involves distal and 5' non-coding sequences that are conserved between mouse and pig

The 5' flanking regions of the mouse and pig urokinase plasminogen activator (uPA) genes were sequenced and sequence homology interrupted by repeat elements was found to extend to -4.6kb in pig and -6.6kb in mouse. A transient transfection procedure was devised for the murine macrophage cell line RAW264. Pig uPA promoter-CAT constructs were more active than mouse constructs in this assay. This contrast may involve sequence differences within 100 bp of the transcription start site. The selective deletion of distal regions of the promoter (greater than 2.6 kb upstream), and of a conserved element, 5'-AGGAGGAAATGAGG-TCA-3' around -2 kb greatly reduced the activity of reporter constructs in RAW264 cells. Electrophoretic mobility shift assays using the latter sequence identified a single nuclear protein complex. This element has been referred to as PEA3/AP1-like, but the complex did not comigrate with either AP1 or known proteins that bind polypurines (including the macrophage-specific factor PU-1) and was not competed by AP1 or polypurine oligonucleotides. uPA promoters contain multiple AP1 and AP2-like DNA sequences, which were recognised by nuclear proteins expressed constitutively in RAW264 cells. They also contain multiple binding sites for NF kappa B but activated NF kappa B was not expressed in RAW264 cells. The conserved, transcribed 5' non-coding sequences were also required for maximal gene expression. Hence, the uPA promoter contains multiple weak cis-acting elements distributed over 7.0 kb 5' to the translation start site.

Identification of an A-factor-dependent promoter in the streptomycin biosynthetic gene cluster of Streptomyces griseus

A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone) is a microbial hormone controlling streptomycin (Sm) production, Sm resistance and sporulation in Streptomyces griseus. In order to identify A-factor-dependent promoters in the Sm biosynthetic gene cluster, a new promoter-probe plasmid with a low copy number was constructed by using an extremely thermostable malate dehydrogenase gene as the reporter. Of the three promoters in the Sm production region that includes strR, aphD and strB, only the promoter of strR, which codes for a putative regulatory protein, was found to be directly controlled by A-factor. This was also confirmed by S1 nuclease mapping. The region essential for its A-factor-dependence was determined to be located 430-330 base pairs upstream of the transcriptional start point. Increase in the copy number of the strR promoter region did not lead to a corresponding increase in the total promoter activity, probably due to titration of a putative activator which binds to the enhancer-like region and controls the expression of the strR promoter. This putative activator is apparently distinct from the A-factor-receptor protein. The aphD gene, which encodes the major Sm resistance determinant, Sm-6-phosphotransferase, was transcribed mainly by read-through from the A-factor-dependent strR promoter; this accounts for the prompt induction of Sm resistance by A-factor.