Expression in Bacillus subtilis of the gene for human urogastrone using synthetic ribosome binding sites

Production of functional active human growth factors in insects used as living biofactories

Growth factors (GFs) are naturally signalling proteins, which bind to specific receptors on the cell surface. Numerous families of GFs have already been identified and remarkable progresses have been made in understanding the pathways that these proteins use to activate/regulate the complex signalling network involved in cell proliferation or wound healing processes. The bottleneck for a wider clinical and commercial application of these factors relay on their scalable cost-efficient production as bioactive molecules. The present work describes the capacity of Trichoplusia ni insect larvae used as living bioreactors in combination with the baculovirus vector expression system to produce three fully functional human GFs, the human epidermal growth factor (huEGF), the human fibroblast growth factor 2 (huFGF2) and the human keratinocyte growth factor 1 (huKGF1). The expression levels obtained per g of insect biomass were of 9.1, 2.6 and 3mg for huEGF, huFGF2 and huKGF1, respectively. Attempts to increase the productivity of the insect/baculovirus system we have used different modifications to optimize their production. Additionally, recombinant proteins were expressed fused to different tags to facilitate their purification. Interestingly, the expression of huKGF1 was significantly improved when expressed fused to the fragment crystallizable region (Fc) of the human antibody IgG. The insect-derived recombinant GFs were finally characterized in terms of biological activity in keratinocytes and fibroblasts. The present work opens the possibility of a cost-efficient and scalable production of these highly valuable molecules in a system that favours its wide use in therapeutic or cosmetic applications.

Production of mouse epidermal growth factor in yeast: high-level secretion using Pichia pastoris strains containing multiple gene copies

We have constructed a synthetic secretion cassette encoding the alpha-factor prepro leader peptide from Saccharomyces cerevisiae fused to mouse epidermal growth factor (mEGF). This was used to compare the secretion of mEGF, a 53-amino acid polypeptide, in S. cerevisiae and Pichia pastoris. In both yeasts the leader sequence was accurately and efficiently cleaved showing that the S. cerevisiae-derived alpha-factor prepro region is correctly recognised and processed in P. pastoris. Of the total mEGF produced, over 90% was exported to the culture supernatant, although the final level of accumulation was dependent on the composition of the growth medium. With P. pastoris there was instability of the protein in minimal medium (yeast nitrogen base), probably caused by extracellular proteases. This was overcome by adding 1% Casamino acids and buffering the medium to pH 6.0. To increase the level of secreted mEGF we have developed a method for rapidly screening large numbers of P. pastoris transformants for the presence of many copies of a foreign gene. Using this procedure we isolated a strain containing 19 integrated copies of the mEGF gene which secreted 450 micrograms/ml of mEGF in high-density fermentations. Characterisation of the yeast-derived mEGF showed the presence of truncated forms, mEGF1-51 and mEGF1-52, as was found with S. cerevisiae-secreted human EGF [George-Nascimento et al., Biochemistry 27 (1988) 797-802]. In addition, the full-length protein, mEGF1-53, was secreted by P. pastoris.

Construction of a new shuttle expression vector for Bacillus subtilis and Escherichia coli by using a polycistronic system

A shuttle vector has been constructed by fusing the Bacillus subtilis trimethoprim-resistance-carrying (TpR) plasmid pNC601 with the Escherichia coli plasmid pBR322. The resultant plasmid pNBL1 can replicate in both B. subtilis and E. coli, conferring Tp resistance on both cells and ampicillin resistance (ApR) on E. coli. The B. subtilis dihydrofolate reductase operon (dfr) on pNC601 and therefore on pNBL1 consists of the thymidylate synthase B gene (thyB) and the TpR-dihydrofolate reductase gene lacking the C-terminal seven codons (designated as drfA' as compared with the complete dfrA gene). A direct-expression vector pNBL3 has been constructed by inserting synthetic oligodeoxynucleotides containing a Bacillus ribosome-binding site (RBS) and the ATG codon downstream from dfrA' on pNBL1. When the E. coli lacZ gene was placed downstream from the dfrA' gene in pNBL3, efficient synthesis of beta-galactosidase was observed in both cells, showing that the polycistronic expression system is suitable for directing expression of heterologous genes. Translational efficiency of the lacZ gene on pNBL3 was further examined in B. subtilis by changing the sequence upstream from lacZ. Unlike the results previously reported [Sprengel et al., Nucleic Acids Res. 13 (1985) 893-909], when RBS was present, the high level of lacZ expression was preserved irrespective of spacing between the stop codon of the upstream dfrA' gene and the start codon of the downstream lacZ gene. However, in the absence of RBS, the spacing between both genes affected lacZ expression. That is, translational coupling of dfrA'-lacZ was observed, although the translational efficiency was very low.

Use of a portable ribosome-binding site for maximizing expression of a eukaryotic gene in Escherichia coli

To maximize expression of a eukaryotic gene in Escherichia coli, a series of plasmids were constructed containing various synthetic ribosome-binding sites (RBS). These sites consist of a Shine-Dalgarno (SD) region (with translation stop codons in all three reading frames) positioned at distances 5-9 nucleotides (nt) from the AUG initiator codon of the gene coding for human T-cell growth factor (TCGF or IL-2). The region encompassing the RBS through the TCGF structural gene from each of these plasmids was inserted as a 'cassette' into seven different E. coli expression vectors, and TCGF production was measured. Our results demonstrate a greater than 2000-fold range of TCGF synthesis dependent upon the promoter and the synthetic RBS used. The translational efficiency of the TCGF gene was found to be influenced by the quality of the RBS, which is in part determined by the external sequence context of this site. The synthetic RBS, containing the necessary information for the translation initiation process, readily accessible by restriction sites, should be of general usefulness in obtaining maximum expression of eukaryotic genes in E. coli.

Direct expression of urogastrone gene in Escherichia coli

Human epidermal growth factor (urogastrone; UG) is a 53-amino acid polypeptide hormone. A 192-bp DNA fragment containing the coding sequence for methionyl UG (Met-UG) and the ribosome-binding site (RBS) was chemically synthesized and placed downstream from the promotor for the Escherichia coli outer-membrane lipoprotein gene (lpp) on a plasmid. E. coli cells harboring the plasmid directed the synthesis of Met-UG at 10(2)-10(3) molecules per cell. Next, the coding sequence for Met-UG was inserted in a runaway-replication plasmid and expressed under the control of the lpp promoter and the RBS derived from bacteriophage Mu cII gene. Upon heat induction, the cells harboring the recombinant plasmid synthesized 10(5) molecules of Met-UG per cell.

A promoter for Bacillus subtilis expression vector

A 117-bp EcoRI-PstI fragment with strong promoter activity (P1 promoter) was cloned from Bacillus subtilis chromosomal DNA and sequenced. The P1 promoter was shown to contain a putative -35 region (TTTACT) and -10 region (TAGATT), and promotes expression of cloned human interleukin-2 (IL-2) and human interferon-gamma (IFN-gamma) genes in B. subtilis.