A survey of vectors for regulating expression of cloned DNA in E. coli

Optimization of recombinant gene expression in Escherichia coli

The major targets for improvement of recombinant expression efficiency in Escherichia coli are gene dosage, transcription and, to some extent, translation. In order to evaluate the relative importance of these factors, the kinetics of specific mRNA compared to product formation was studied for different widely used expression systems, producing recombinant human superoxide dismutase. For a system employing phage T7 RNA polymerase, where a high level of recombinant protein expression puts a high metabolic burden on the cells, it was shown that transcription is not the limiting factor. To improve the translation rate of a common vector based on the tac promoter, the Shine-Dalgarno (SD) sequence was mutated towards stronger homology to the anti-SD sequence of the E. coli 16S rRNA. A 12.2-fold increase in protein yield was accompanied by a 4.3-fold increase in specific mRNA, indicating that transcription of the recombinant gene is coupled to translation. As this coupling amplifies the detrimental effect of a low-efficiency ribosomal binding site, much attention should be paid to translation initiation when optimizing a recombinant protein production system. Finally, reasons for the high expression level before induction are discussed, and first results towards reducing it are presented.

Expression vectors for the use of eukaryotic luciferases as bacterial markers with different colors of luminescence

An easy way to identify microorganisms is to provide them with gene markers that confer a unique phenotype. Several genetic constructions were developed to use eukaryotic luciferase genes for bacterial tagging. The firefly and click bettle luciferase genes, luc and lucOR, respectively, were cloned under constitutive control and regulated control from different transcriptional units driven by P1, lambda PR, and Ptrc promoters. Comparison of the expression of each gene in Escherichia coli cells from identical promoters showed that bioluminescence produced by luc could be detected luminometrically in a more sensitive manner. In contrast, luminescence from intact lucOR-expressing cells was much more stable and resistant to high temperatures than that from luc-expressing cells. To analyze the behavior of these constructions in other gram-negative bacteria, gene fusions with luc genes were cloned on broad-host-range vectors. Measurements of light emission from Rhizobium meliloti, Agrobacterium tumefaciens, and Pseudomonas putida cells indicated that both luciferases were poorly expressed from P1 in most bacterial hosts. In contrast, the lambda promoter PR yielded constitutively high levels of luciferase expression in all bacterial species tested. PR activity was not regulated by temperature when the thermosensitive repressor cI857 was present in the bacterial species tested, except for E. coli. In contrast, the regulated lacIq-Ptrc::lucOR fusion expression system behaved in a manner similar to that observed in E. coli cells. After IPTG (isopropyl-beta-D-thiogalactopyranoside) induction, this system produced the highest levels of lucOR expression in all bacterial species tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping of viral epitopes with prokaryotic expression products

Several systems are available for the expression of foreign gene sequences in Escherichia coli. We describe the use of prokaryotic expression products of viral gene fragments in order to identify the regions that specify the binding sites of antibodies. This approach is particularly successful if the antigenicity does not depend on the native protein, but only on the amino acid sequence, i.e., if the epitope is sequential. Combining prokaryotic expression with the use of synthetic peptides often permits a fast and accurate mapping of an epitope. The occurrence of immunodominant sequential epitopes on the surfaces of viruses seems to be a widespread phenomenon.