MATERIALS AND METHODS FOR BIOSYNTHESIS OF SERINE AND SERINE-RELATED PRODUCTS

Abstract: DNA encoding 3-phosphoglycerate dehydrogenase (PGD) with reduced sensitivity to inhibition by serine in comparison to wild-type PGD - i.e., the DNA encodes PGD which has at least some level of enzymatic activity useful for biosynthesis, and which retains that activity at higher serine levels than does the (unmodified) wild-type PGD. Also disclosed are: a) PGD having the amino acid sequence of the above-described engineered DNA; b) expression vectors comprising the engineered DNA and regulatory DNA positioned and oriented for expression of the engineered DNA in a host expression system; c) cells comprising such expression vectors; and d) methods for producing serine or a serine-derived product by culturing such cells.

Description: MATERIALS AND METHODS FOR BIOSYNTHESIS OF SERINE AND SERINE-RELATED PRODUCTS I. Background of the Invention This invention relates to the general field of biosynthesis of serine and products related to serine, particularly tryptophan, and to methods and materials used in that biosynthesis. Serine is a primary intermediate in the biosynthesis of a wide variety of cellular metabolites including such economically important compounds as choline, glycine, cysteine and tryptophan. In addition, serine acts as a single carbon donor and is responsible for 60 - 75% of the cell's total need for C- [ _ units through the production of 5,10-methylenetetrahydrofolate from tetrahydrofolate. These Ci units are used in a wide variety of biosynthetic pathways including the synthesis of ethionine, inosine onophosphate, other purines and some pyrimidines (e.g. thy idine and hydroxymethyl cytidine) . The serine biosynthetic pathway shown in Fig. 1 is generally available to a wide variety of tissues and microorganisms. The first committed step in that pathway is the conversion of 3-phospho-D-glyceric acid (PGA) to 3- phosphohydroxypyruvic acid (PHA) by means of the enzyme 3- phosphoglycerate dehydrogenase (PGD) . The gene encoding PGD has been cloned and sequenced, and the amino acid sequence of the PGD subunit has been deduced. Tobey and Grant, J. Biol. Chem. 261:12179-12183 (1980). In procaryotes (particularly bacteria) and microorganisms such as yeast, but not in higher eukaryotes, activity of wild-type PGD is inhibited by cellular serine levels. This inhibition has been studied kinetically and reportedly proceeds in an allosteric matter. Tobey and Grant, J. Biol. Chem. , 261:12179-12183 (1986) ; Dubrow and Pizer, J. Biol. Chem. 252:1527-1551 (1977); McKitrick and Pizer, J. Bacteriol. 141:235-245 (1980). Tosa and Pizer, J. Bacteriol. 106:972-982 (1971) studied the effect of a normally toxic serine analog, L-serine hydroxamate, on an E. coli strain. Selection on a growth medium containing that analog yielded serine resistant mutants. Some mutants were shown to have a modification in an enzyme unrelated to PGD, seryl-tRNA synthetase. Crude extract of one mutant showed PGD activity with reduced serine sensitivity (See J. Bacteriol. 106:972-982 (1971); Fig. 5; Table 6; and see p. 973 bottom left col., p. 977 bottom left col.) . II. Summary of the Invention One aspect of the invention generally features DNA encoding 3-phosphoglycerate dehydrogenase (PGD) with reduced sensitivity to inhibition by serine in comparison to wild- type PGD—i.e., the DNA encodes PGD which has at least some level of enzymatic activity useful for biosynthesis, and which retains that activity at higher serine levels than does the (unmodified) wild-type PGD. In preferred embodiments, the wild-type PGD is microbial or yeast PGD. Also preferably, the engineered DNA encodes PGD which comprises an alteration in the C-terminal 25% of wild- type PGD, preferably in the C-terminal 50 aminoacids. For example, the engineered DNA may encode PGD comprising a deletion in part or all of the C-terminus. Also preferably, the engineered DNA encodes PGD having an insertion in the C- terminus (e.g., between VAL363 and ASN364, or between ALA392 and GLN394) in addition to the deletion described above, or as a separate alteration. The invention also features: a) PGD having the amino acid sequence of the above-described engineered DNA; b) expression vectors comprising the engineered DNA and regulatory DNA positioned and oriented for expression of the engineered DNA in a host expression system; c) cells comprising such expression vectors; and d) methods for producing serine or a serine-derived product by culturing such cells. As to c) , above, the cell preferably is deleted for wild-type serA. Yet another embodiment generally features a cell engineered (e.g., it includes a reco binant genetic construction) to produce a PGD-encoding RNA transcript with an altered 3' end which transcript is translated by the cell to yield PGD with reduced sensitivity to inhibition by serine in comparison to wild-type PGD. The invention provides decontrol of an important biosynthetic control point, thereby enhancing production of numerous compounds downstream of that point, including, in particular, serine and serine-derived products such as tryptophan. Other cellular metabolites derived from serine (i.e., serine is a primary intermediate in their biosynthesis) include choline, glycine, cysteine and c-_- donor-dependent compounds such as methionine, inosine monophosphate, purines, and some pyri idines (e.g., thy idine and hydroxymethyl cytosine) . III. Description of the Preferred Embodiments A. Drawings Fig. 1 depicts steps in the biosynthesis of L-serine from glucose. Fig. 2 is the sequence of the E. coli serA gene reported by Tobey and Grant (cited above) and the amino acid sequence deduced from the gene. Fig. 3 depicts bioconversion of L-serine and tetrahydro- folate to glycine and N 5 ,N 10 -methylene tetrahydrofolate. B. Providing Serine-Insensitive PGD 1. Genetically engineered constructions. The preferred embodiments of the invention feature biosynthesis of serine and serine-related products—e.g., products described above derived by biosynthesis from serine. A first step in biosynthesis of these compounds according to the invention is the provision of serine insensitive PGD, as discussed below. We have determined that there is a specific serine feedback mediating domain in PGD, and that domain can be altered to reduce serine sensitivity while maintaining useful levels of PGD activity. Fig. 2 shows one particular PGD genetic and amino acid sequence which can be used for reference in the following discussion. The sequence of Fig. 2 includes 410 amino acids (including the initial Met which is cleaved from the mature protein) . The domain of PGD that can reduce serine sensitivity, without destroying PGD activity, is within the C-terminal 25% of the molecule, most preferably the 50 C-terminal residues. Examples of PGD modifications that fall within the invention are deletions of some or all of the C-terminal 42 amino acids, or insertions or substitutions within that region which reduce serine sensitivity while retaining useful PGD function. For example, insertion of amino acid residues between Val 363 and Asn 364 will increase the Kj_ of the PGD over wild-type, while retaining PGD activity. More dramatic increases in K_ are accomplished by deleting some or all of the C-terminal amino acid residues. For example, deletion of the C-terminal residues GTIRARLLY and replacement with ASLD increases j by several orders of magnitude, while retaining a useful level of PGD activity. Other insertions within the scope of the invention are insertions between Ala392 and Gln394. Other useful modifications include deletions from the C- terminus in addition to the insertions and modifications discussed above. Genes encoding serine-insensitive PGD described above can be constructed by genetic engineering techniques that involve altering the 3' end of the coding region coding for the C- ter inal amino acids, and then transforming a host strain with a vehicle to express the altered PGD enzyme. Candidate altered enzymes are screened (as described below) for serine affinity (K j _) and for PGD activity by the methods generally discussed below. 2. Screening the genetically engineered constructions. In screening genetic constructions made by the above- described methods, the following assays of PGD activity and of serine sensitivity are used. While not critical to the invention r the assay of PGD activity is generally necessary in order to establish the degree of serine sensitivity of the altered enzyme. As is well known in the art, enzyme activity is a funtion of the total number of enzyme molecules and the catalytic activity of each molecule. Thus, in comparing the catalytic activity of PGD feedback variants, steps must be taken to adequately control for the relative number of PGD molecules for samples in which relative catalytic activity is to be compared. There are a number of ways in which this may be accomplished. However, since it is difficult to adequately establish the level of gene expression in cells transformed with truncated serA genes (due to decreased viability) , the most suitable way to compare PGD activity produced from various constructs and the wild type is to chromosomally integrate the altered serA gene containing standard regulatory elements in a single copy, followed by harvesting the transformants and determination of the relative catalytic activity as compared to PGD from wild type cells. Any method suitable for the measurement of PGD activity may be employed. PGD activity may be measured through detection of either the forward or the reverse reaction by the method of McKitrick, John C. and Lewis I. Pizer. (1980) J. Bacteriol. 141 235-245. The enzymatic assay described above is suitable for determination of serine sensitivity for any PGD enzyme, including those with chemically modified C-termini. The assay is performed in the presence of various levels of serine. The catalytic activity in the presence of serine is compared to catalytic activity in the absence of serine, and the Kj_ calculated. In most cases it will be preferred to reduce serine sensitivity without significantly altering PGD catalytic activity. In still other embodiments it may be desirable to reduce both the feedback sensitivity and the catalytic activity. The constructions having a C-terminal amino acid sequence of 3-phosphoglycerate dehydrogenases listed in Table 1 (described below) may be used. Table 1 C-terminal Amino Acid Sequences of 3-phosphoglycerate dehydrogenases SerA Sequence I ki/μinl Units WT I AEQG V -NIA AQYL QTSA QMGY VVID IEAD EDVAEKA —QA.MKAI PGT- -IRA RLLY 1455 I AEQG V -NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL --QA MKAI PASL D 1459 I AEQG VLV 1507 I AEQG VL 1508 I AEQG VCSR ANIA AQYL QTSA QMGY VVID IEAD EDVAEKAL --QA MKAI PGT- -IRA RLLY 1509 I AEQG V -NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL SRQA MKΛJ. PGT- -IRA RLLY 1510 I AEQG V -NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL L 1511 I AEQG V -NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL --QA MKAI PGT- AIRA RLLY 1512 I AEQG V NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL --QA MKAI PVL 1530 I AEQG V -CSR AIRA RLLY 1531 I AEQG V NIA AQYL QTSA QMGY VVID IEAD EDVAEKAL SR AIRA RLLY I Other constructions with modified 3' ends also fall within the scope of the present invention since it is a simple matter to prepare test constructs and transform cells according to the present invention and test for serine inhibition of PGD activity. Any vector which leads to expression of a PGD protein lacking sensitivity to inhibition by serine pertains to the present invention. In general, however, in the absence of a sink for serine, high levels of expression of feedback free PGD should be avoided since the resulting high cytoplasmic levels of serine or serine-derived metabolites can be toxic to the cell. Thus, in general, for any construct coding for a feedback inhibited PGD with normal catalytic activity and expression levels similar to those from the native gene, transformation will likely lead to high levels of PGD expression and decreased cellular viability. The toxicity of high levels of serine produced may in fact select for mutants with decreased PGD expression. Thus while transformation using multi-copy plasmids may be useful in initial screening of constructs with some embodiments, it is preferred to chromoso ally integrate serA constructs in single copies into the genome. Additionally, chromosomal integration as described below facilitates activity measurement of the feedback deleted PGD. Thus, in most embodiments where strong catalytic activity is expected or desired, it is preferred to utilize vectors suitable for single copy chromosomal integration. Many such vectors and strategies for their use are known in the art. See, e.g., Back an, U.S.S.N 07/091,837, filed September 1, 1987, hereby incorporated by reference. Useful vectors and constructs can be made to allow for the successful transformation and expression of the enzyme in an appropriate host for producing the desired product. Means for accomplishing these ends are well known to those familiar with the art and are not central to the present invention. In addition to the altered PGD-encoding DNA, the expression vector will contain various other elements described below. First, the coding sequences present on the vector will be accompanied by the appropriate regulatory elements necessary for the appropriate level of expression of the coding sequences, including promoters, ribosome binding sites, and termination sequences. In most cases, the native serA regulatory sequences will be the preferred source of the catalytically active part of the molecule, although it is recognized that many other regulatory sequences known to the art or yet to be discovered may be employed. Second, it is preferred that sequences encoding selective markers and/or reporter genes, along with the appropriate regulatory elements, will also be present on the vector. The expression of such selective markers is useful in identifying transformants. Appropriate selective marker genes include those coding for ampicillin, tetracycline, and chloramphenicol. Third, the desirability of an origin of replication on the plas id vector depends largely on the desirability of maintaining the genes chromosomally or extrachromosomally. Those familiar with the art appreciate the various strategies by which the lack of an origin of replication can be exploited to promote integration into the chromosome. See, e.g., Back an et al., U.S. Patent 4,743,546, hereby incorporated by reference. Once the expression vector is constructed, a suitable host cell can be transformed with a vector containing a transcription unit coding for a serine insensitive PGD protein. In most cases, it is useful to employ cells for which the endogenous PGD protein is known to be inhibited by serine and in which the endogenous serA gene is deleted and replaced by the altered gene of the invention. Such cell systems are useful for the overproduction of serine-related metabolites. Cells known to contain serine sensitive proteins are prokaryotes and yeasts. The following example illustrates, but does not limit, the invention. Example 1 Construction of serA Gene Alleles Encoding Feedback Resistant 3-phosphoglycerate Dehydrogenases The E. coli K12 serA gene was isolated on a 6.4 Kb DNA fragment from a Sau3A partial digest cloned into the Bell site of pTR264. See, Back ann et al., U.S.S.N. 07/285,128, filed 12/16/88 and Roberts et al. , Gene 12:123 (1980). This plasmid was named pKB1302. A 3 Kb Sail to SphI fragment of pKB1302 DNA containing the serA gene was cloned into pUC19 to generate pKB1321. pKBl370 was generated by cloning a 3 Kb Hindlll to Sail fragment containing the serA gene into PBR322. Alleles of serA encoding feedback resistant 3- phosphoglycerate dehydrogenases were generated by inserting Xbal linkers at restriction sites in the 3' region of the serA gene. A partial digest of plasmid pKB1321 by Hindi yielded blunt ends at position 1793, where insertion of linkers gave: a) pKB1459, encoding a truncated 3- phosphoglycerate dehydrogenase; b) pKB1507, encoding a truncated 3-phosphoglycerate dehydrogenase; and c) pKB1508 which encodes a 3-phosphoglycerate dehydrogenase with a four amino acid residue insert. PstI digestion of pKB 1321 gives a 3 ' overhang at position 1888. Blunt ends were generated by the action of the Klenow fragment of DNA polymerase I. Linkers were ' ligated into the blunt end fragments and the derived plasmids were pKB1509 which encodes a 3-phosphoglycerate dehydrogenase with a two- a ino-acid insert and pKB1510 which encodes a truncated 3- phosphoglycerate dehydrogenase. A Kpnl digest of pKB1370 was made blunt ended with Klenow fragment of DNA polymerase I and inserted linkers yielded plasmids encoding truncated 3- phosphoglycerate dehydrogenase, pKB1455 and pKB1512, or 3- phosphoglycerate dehydrogenase with a two amino acid residue insert, pKB1511. Deletion plasmids pKB1530 and pKB1531 were generated by inserting the 0.8 Kb BamHI to Xbal fragment from pKB1508 or the 0.9 Kb BamHI to Xbal fragment from p B1509 respectively, into the 5.8 Kb BamHI to Xbal fragment of pKB1511. The following Table 2 summarizes the various constructs made. Table 2 SerA Plasmid Restriction Linker Result Allele Site Ser A 1455 pKB 1370 Kpn I CTAGTCTAGACTAG Ser A 1459 pKB 1321 Hind II CTAGTCTAGACTAG Ser A 1507 pKB 1321 Hind II CTCTAGAG Ser A 1508 pKB 1321 Hind II TGCTCTAGAGCA Ser A 1509 pKB 1321 Pst I GCTCTAGAGC Ser A 1510 pKB 1321 Pst I TGCTCTAGAGCA Ser A 1511 pKB 1370 Kpn I GCTCTAGAGCA Ser A 1512 pKB 1370 Kpn I TGCTCTAGAGC Ser A 1530 pKB 1511 + pKB 1508 Hind II + Kpn I Ser A 1531 pKB 1511 + pKB 1509 Pst I + Kpn I For all the constructs, the starting vector, the restriction site used, and the sequence of the inserted linker are indicated. The Ki values for serine are given in Table 1 as well as the relative catalytic activity for three of these constructs following chromosomal integration (described below) . N/A indicates that the construction was not chromosomally integrated, and the activity level therefore was not standardized. 3. Chemical Modifications Those skilled in the art will understand that deletions or modifications of the C-terminus of wild-type PGD can be accomplished enzymatically or chemically, e.g. by various carboxypeptidases including carboxypeptidase Y or by lactoperoxidase mediated iodination. 4. Use of Antisense mRNA Alternatively, it may be possible to reduce serine sensitivity in vivo through the generation of PGD-encoding transcripts truncated at the 3 ' end by means of the producing antisense RNAs that include nucleotide sequences complementary to portions of the 3' coding region of native or transformed PGD coding sequences. C. Production of Desired Compounds As shown in Fig. 3, serine is an intermediate in the production of glycine. It is also an intermediate in the production of N 5 ,N 10 -methylenetetrahydrofolate which is generalized C;L donor essential for synthesis of methionine, purines (including inosine) and some pyrimidines. Thus, the over-production of serine from phosphoglycerate may be useful in a wide range of bacterial production systems including production systems for choline, glycine, cysteine, methionine, tryptophan, and all purines including inosine monophosphate. The following specific examples illustrate the invention. Example 2 Host Strain Preparation Sequences internal to a plasmid born serA gene were replaced with a kana ycin resistance gene. This plasmid was then used to inactivate the host strain serA gene by means of allele exchange as follows: The serA region of YMC9 (ATCC33920) was cloned from chromosomal DNA, partially digested with Sau 3AI, by complementation of PC1523 (argI61, argF58, serA27, purA54, thr-25, tonA49, relAl, spoTl) , obtained from Coli Genetic Stock Center, Yale University, New Haven, CT. A 3 kb fragment carrying the serA gene was subcloned into pUC19 giving rise to a plasmid called pKB1321. From this plasmid a 3kb Sail to Hindlll fragment was recloned into pBR322 giving rise to plasmid pKB1370. The Kpnl site at the 3' end of the serA gene was converted to BamHI with a linker and the BamHI fragment internal to the resulting serA was replaced with the BamHI fragment from pUC-4-KSAC (Pharmacia) containing the Tn903 kanamycin resistance gene. This new plasmid was designated pKB1429. A pBR322 derivative called pKB 701 (ATCC 39772), see USSN 06/757,019 hereby incorporated by reference, was generated in which the Mbol and TThlll 1 flanking the origin of replication were converted to Kpnl sites. The Sail to EcoRI fragment containing serA::KanR from pKB1429 was cloned into pKB701 giving rise to pKB1438. pKB1438 was digested with Kpnl to remove the ori region. The large fragment containing the ampicillin resistance coding region as well as the serA::KanR was circularized and used in a CaCl2 transformation of YMC9. Following transformation, the host YMC9 cells were placed under selection on ampicillin. Under these conditions, ampicillin resistant clones develop by incorporation of the circular DNA through homologous recombination in the serA gene flanking regions. Growth of the ampicillin resistant isolate in the absence of ampicillin selection results in loss of the ampicillin resistance gene by homologous recombination of the repeated sequences of serA gene flanking regions. Such strains were identified by the loss of production of β-lactamase using AmpScreen (BRL) according to the manufacturer's directions. Duplicate streaking of single colonies on media in the presence and absence of serine revealed ampicillin sensitive clones requiring serine for growth on minimal medium and which were also resistant to kanamycin. One such isolate was named KB875. Example 3 Chromosomal Integration of Altered serA Sequences by Allele Exchange The serA1455 allele was introduced to the chromosome by a process analogous to that used for the introduction of serA: :KanR as outlined in example 2. Briefly, a fragment (Sail to Hindlll) bearing the serA1455 allele was cloned into pkB701. The plasmid origin was removed by Kpnl digestion. The circularized DNA was used to tranfor to ampicillin resistance giving rise to a strain designated. After non-selective growth, using Ampscreen and replica plating for kanamycin, KB904 (serA1455) was isolated and shown to be sensitive to ampicillin and kanamycin KB904. The resulting serA1544 allele can be transferred into production strains by PI transduction. Miller (1972) Experiments In Mol. Genetics Cold Spring Harbor Press, pp. 201-205. Example 4 Chromosomal Integration of Altered serA Sequences by recD Dependent Gene Replacement Another approach was utilized to move the serA1508 allele on to the chromosome. The strain KB875 was made recD by PI transduction from V220 (recD, argA:TnlO. Amundsen et al., (1986) Proc. Acad. Sci., U.S.A. 82 5558-5562) (DSM 6823). The gene for an essential third subunit of exonuclease V. to give JGP101. The plasmid pKB1508 was linearized and used to transform JGP101 to serine prototrophy essentially as described by Shevell et al., (1988) J. Bacteriol. 170 3294- 3296, to give JGP103. The serA1508 allele can then be moved to production strains by Pi transduction. Miller et al., Experiments in Mol. Genetics Cold Spring Harbor Lab. , pp. 201-205 (1972) . Harvesting of Overproduced Metabolites For the overproduction of serine-related metabolites, cells can be prepared which produce PGD with reduced serine sensitivity, and grown in fermentors under the appropriate conditions, in most cases to stationary phase. The cells will then be harvested and lysed and the desired metabolite prepared according to standard biochemical procedures. Conditions, principles, and references for the growth of microbes, and the harvesting of specific metabolites are provided by Crueger and Crueger (1982) (Biotechnology: A Textbook of Industrial Microbiology) and Herrmann and Somerville (1983) (Amino Acids: Biosynthesis and Genetic Regulation) incorporated herein by reference.

Claims: Claims : 1. Engineered DNA encoding 3-phosphoglycerate dehydrogenas (PGD) with reduced sensitivity to inhibition by serine in comparison to wild-type PGD. The engineered DNA of claim 1 in which the wild-type P is microbial or yeast PGD. The engineered DNA of claim 1 in which said DNA encodes PGD comprising an alteration in the C-terminal 25% of wild-type PGD. The engineered DNA of claim 3 in which said DNA encodes PGD comprising an alteration in the C-terminal 50 amino acids of wild-type PGD. 5. The engineered DNA of claim 3 in which said DNA encodes PGD comprising a C-terminal deletion of wild-type PGD. 6. The engineered DNA of claim 3 in which said C-terminal alteration comprises an insertion into a wild-type PGD sequence. The engineered DNA of claim 6 wherein said insertion is between VAL 363 and ASN 364 or between ALA 392 and GLN 394 of wild-type PGD. ■ 8. The engineered DNA of claim 1 selected from the group consisting of: a) serA 1455 f) serA 1510 b) serA 1459 g) serA 1511 c) serA 1507 h) serA 1512 d) serA 1508 i) serA 1530 e) serA 1509 j) serA 1531 A 3-phosphoglycerate dehydrogenase having the amino aci sequence encoded by the engineered DNA of any one of claims 1-8. 10. An expression vector comprising the engineered DNA of any one of claims 1-8 and regulatory DNA positioned and oriented for expressing of said engineered DNA in a hos expression system. 11. A cell comprising the engineered DNA of any one of claims 1-8 and regulatory DNA positioned and oriented to express said engineered DNA in said cell. 12. The cell of claim 11 in which said cell is deleted for wild- ype serA. 13. A method for producing a desired product which is serine or serine-derived, comprising culturing a cell according to claim 11 and recovering said desired product. 14. The method of claim 13 in which said product is serine. 15. A cell engineered to produce a PGD-encoding transcript with an altered 3' end, said cell being capable of translating said transcript into PGD with reduced sensitivity to inhibition by serine in comparison to wild-type PGD.

Inventors: BURLINGAME RICHARD P, BURLINGAME, RICHARD, P.
Applicants: WACKER CHEMIE GMBH, WACKER-CHEMIE GMBH
Application year: 1992
Main classification: C12P13
IPC codes: C12N1, C12N9, C12N15, C12P13, C12R1