Chemistry: molecular biology and microbiology – Process of mutation – cell fusion – or genetic modification – Introduction of a polynucleotide molecule into or...
Reexamination Certificate
1999-06-25
2001-09-18
Schwartzman, Robert A. (Department: 1635)
Chemistry: molecular biology and microbiology
Process of mutation, cell fusion, or genetic modification
Introduction of a polynucleotide molecule into or...
C435S006120, C435S069100, C435S091100, C435S091420, C435S252330, C435S320100, C536S023100, C536S024100
Reexamination Certificate
active
06291245
ABSTRACT:
BACKGROUND OF THE INVENTION
Known and typical prokaryotic expression plasmids contain, in addition to one or several antibiotic resistance gene(s), additional DNA sequences which are not required and burden the cell metabolism. These include DNA sequences resulting from the cloning process, DNA segments that are relics from multipurpose vectors such as specific promoters for the in vitro synthesis of mRNA and specific phage replication origins for the synthesis of single stranded DNA and extend to rudimentary duplicated vector sequences which can be the cause of undesired plasmid rearrangements.
The presence of a plasmid and in particular of an expression vector is an additional metabolic burden for the cell. This results in a selection pressure which favours the formation of cells without plasmids. One method of selecting for cells containing plasmids is antibiotic selection.
Antibiotics such as ampicillin , tetracycline, kanamycin and chloramphenicol are usually used for selection. The use of &bgr;-lactam antibiotics such as ampicillin is especially problematic in the production (fermentation) of therapeutic products.
For the reasons discussed above antibiotic plasmid selection, especially by means of &bgr;-lactam antibiotics, is not favored in recent methods. In some cases a host/vector system has been developed that proved to be so stable that it was possible to omit a plasmid selection during the preculture and during the main fermentation (Weir, A. N. C. and Mountain, A., EP 0651803). However, as a rule this is associated with a reduced product yield. In cases in which antibiotic selection is indispensable, tetracycline is often used as an alternative to ampicillin (Carter, P. et al., 1992). In contrast to the &bgr;-lactam antibiotics, the tetracyclines are not reactive chemical compounds and cannot be inactivated by enzymatic modification during fermentation. The tetracycline resistance gene codes for a protein which modifies the bacterial membrane and thus prevents the antibiotic from entering the cell.
Selection systems have been developed that do not use antibiotic resistance, instead exploiting complementation technology. Struhl, K. and coworkers (Struhl, K. et al., 1976) using imidazole glycerol phosphate dehydratase (HIS3) as an example, show that an appropriate
E. coli
mutant (hisB) can be directly complemented by means of plasmids containing yeast DNA and that the yeast enzyme coded by HIS3 is functionally expressed in
E. coli
. This ability of genes to complement mutations in
E. coli
was used as a selection criterion to clone for example (complementation cloning) other yeast genes (LEU2, URA3, TRP5 and ARG4).
E. coli
host strains with a stable mutation (reversion rate >10
−10
; preferably non-revertable deletion mutants) are required for selection by complementation. However, the reversion rate of known mutations is of the order of magnitude of <10
−10
(Schlegel, H. G., 1981).
The known
E. coli
laboratory strains differ with regard to their genotype in individual mutations which in many cases were produced by undirected mutagenesis by radiation (X-ray or UV radiation), chemical mutagens (e.g. base analogues, nitrous acid and alkylating compounds) or biological techniques (e.g. phage Mu and transposon mutagenesis (Bachman, B. J., 1986).
SUMMARY OF THE INVENTION
We have developed a stable prokaryotic (preferably
E. coli
) host/vector system (expression system) which avoids plasmid selection by antibiotics. The selection principle is based on complementation of a stable auxotrophy of the prokaryotic host cell by an adequate yeast gene.
The invention concerns a minimal prokaryotic expression vector which cannot be homologously recombined with the genome of prokaryotic organisms. The vector contains:
a) an origin of replication,
b) an auxotrophy marker gene,
c) a promoter which is functionally active in prokaryotes and
d) a foreign gene (foreign sequence) to be expressed under the control of the said promoter.
Therefore the invention concerns new prokaryotic expression systems which enable antibiotic-free selection and their use for the production of recombinant proteins. The invention is directed to a prokaryotic expression vector which comprises:
a) An origin of replication;
b) at least one eukaryotic auxotrophy marker gene under the control of a eukaryotic promoter;
d) a foreign gene under the control of a prokaryotic promoter; and
e) one or more transcription terminators.
It is preferable that the prokarytic expression vector is not sufficiently large, in order to reduce the metabolic burden of the cell. The vector element used should preferably be of heterologous origin and very small to engage in homologous recombination with the genome of a prokaryotic host cell. In this regard, a desirable size for the vector of this invention is 2500 base pairs or less, to a lower limit set by the minimum size of an expression vector containing the elements listed above.
A preferred expression vector is assembled such that the auxotrophy marker gene and the foreign gene are oriented in opposite directions with the transcription terminator located between the auxotrophy marker gene and the foreign gene so positioned as to terminate transcription of both genes.
A preferred source of the auxotrophy marker gene of this invention is yeast, for example
Saccharomyces cerevisiae
. Particular marker genes useful in this invention are URA3 or TRP1. Accordingly, the preferred eukaryotic promoter is a yeast promoter. When the URA1 or TRP1 gene is being used, the promoters are part of the 5′ flanking region of the URA3 or TRP1 gene.
Any prokaryotic promoter may be used for the expression of the foreign gene. A preferred prokaryotic promoter is T5-P
N25/03/04
. In the context of this invention, prokaryotic promoter includes promoters derived from bacteriophages.
Any transcription terminator may be used. A preferred transcription terminator is derived from a bacteriophage, for example bacteriophage fd or &lgr;-T0.
Any origin of replication may be used, however preferred are origin of replications from pBR or pUC plasmids.
This invention is also directed to prokaryotic auxotrophic host cells. The host cell of this invention contains a prokaryotic expression vector of this invention and has at least one mutation such that it is unable to express the product expressed by the auxotrophic marker gene in the expression vector. A preferred host cell is an
E. coli
cell, especially a host cell such as an
E. coli
cell which is unable to produce the product of the trpC gene or the pyrF gene. An especially preferred host cell contains a prokaryotic expression vector of this invention.
Such a prokaryotic host cell preferably produces sufficient amounts of the product of the genes URA3 or TRP1 to survive on minimal medium but does not produce an amount of such gene product which constitutes more than 1.0% of the total amount of protein produced by the cell. The URA3 or TRP1 protein produced can be assayed by SDS PAGE analysis cell lysates.
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Grodberg, J. et a
Kopetzki Erhard
Schantz Christian
Epstein William H.
Johnston George W.
Roche Diagnostics GmbH
Schwartzman Robert A.
Zara Jane
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