MICROBIOLOGICAL PRODUCTION METHOD FOR alpha -L-ASPARTYL-L-PHENYLALANINE

Abstract: The present invention relates to a new, microbiological method for the production of alpha -L-aspartyl-L-phenylalanine (Asp-Phe) from the substrates L-aspartic acid (L-Asp) and L-phenylalanine (L-Phe) wherein the substrates are contacted, in the presence of ATP, with a non-ribosomal dipeptide synthetase comprising two minimal modules connected by one condensation domain wherein the N- resp. C-terminal modules are recognising L-Asp and L-Phe, respectively, and the latter module is covalently bound at its N- terminal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4'-phosphopantetheinyl cofactor containing thiolation domain, and that the Asp-Phe formed is recovered. The present invention also relates to novel DNA fragments or combination of DNA fragments encoding a new Asp-Phe dipeptide synthetase, micro-organisms containing such DNA fragments, as well as to the new Asp-Phe dipeptide synthetases itself.

Description: MICROBIOLOGICAL PRODUCTION METHOD FOR -L-ASP RTYL-L-PHENYLALANINE FIELD OF THE INVENTION The present invention relates to a new, microbiological , method for the production of α-L- aspartyl-L-phenylalanme (Asp-Phe) from the substrates L-aspartic acid (L-Asp) and L-phenylalamne (L-Phe) . The present invention also relates to novel DNA fragments or combination of DNA fragments encoding a new Asp-Phe dipeptide synthetase, micro-organisms containing such DNA fragments, as well as to the new Asp-Phe dipeptide synthetases itself. BACKGROUND OF THE INVENTION -L-Aspartyl -L-phenylalamne (hereinafter also referred to as Asp-Phe) is an important dipeptide, inter alia used for the production of α-L-aspartyl-L- phenylalanme methyl ester (hereinafter also referred to as APM) . APM is known to be a high intensity artificial sweetener, having a sweetness which is about 200x as potent as the sweetness of sucrose. The β-form of APM, as well as the stereoisomers of APM wherein one or both of the ammo acids are m the D-configuration, do not have sweet properties. APM is used for the sweetening of various edible materials . Various production methods of APM exist; present routes may toe divided inco chemical and biocnemical/microbiological (m particular, enzymatic) routes. In the ways of producing APM by using known peptide synthesis techniques tedious and expensive processes have to be performed m order to achieve selective -L, L-coupling, involving intensive protecting and deprotectmg of α-ammo, carboxyl and side chain groups. Fermentative routes, on the other hand, m general are cheap and intrinsically they display enantio- and regioselectivity. Therefore, fermentative routes have been considered to be promising alternatives for the above-mentioned chemical and biochemical synthesis routes. As can be seen from EP-A-0036258, it has so far been deemed unsuited to produce the dipeptide Asp-Phe m a micro-organism as part of the microorganism's own protein producing processes; theoretically such production might be achieved by inserting in the DNA of a micro-organism the nucleotide base sequences GAC or GAT (being known to be a codon for L-Asp) and TTT or TTC (being known to be a codon for L- Phe) , preceded and followed by appropriate processing or termination codons m the correct reading frame, and under appropriate control . It therefore has been attempted m EP-A-0036258 to achieve the synthesis of Asp-Phe indirectly through prior production of protein segments of the formula (Asp-Phe) n , where n is a large number; this has been done by inserting into a cloning vehicle a synthesised DNA- fragment coding for such poly- (Asp-Phe) protein. However, such πbosomal fermentative route is still tedious and economically unattractive. Ma]or drawbacks are lying m the recovery of the Asp-Phe dipeptide from the polypeptide. Similar drawbacks can be attributed to a method, as described by Choi, S.-Y. et a . in J. Microbiol. Biotechnol . , 2, 1992, p.1-6, wherein a polypeptide comprising segments of the tripeptide sequence Asp-Phe-Lys is syntnesised. Therefore still need exists for finding a direct fermentative route to Asp-Phe. Direct fermentation of Asp-Phe is hitherto unknown. DESCRIPTION OF THE INVENTION METHOD FOR THE PRODUCTION OF ASP-PHE; Surprisingly, inventors now found a new, and promising alternative microbiological method for the production of α-L-aspartyl -L-phenylalamne (Asp-Phe) from the substrates L-aspartic acid (L-Asp) and L- phenylala ne (L-Phe) wherein the substrates are contacted, m the presence of an effective amount of adenosme-triphosphate (ATP) , with a non-πbosomal dipeptide synthetase comprising two minimal modules connected by one condensation domain wherein the N- termmal module of these modules is recognising L- aspartic acid and the C-termmal module of these modules is recognising L-phenylalamne and is covalently bound at its N-termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' - phosphopantethemyl cofactor containing thiolation domain, and that the α-L-aspartyl-L-phenylalanme (Asp- Phe) formed is recovered. This new method thus provides a microbiological process for direct fermentation of Asp- Phe, which m a subsequent methylation step may be converted into the intense sweetener aspartame. Production of Asp-Phe by direct fermentation is hitherto unknown, as is non-πbosomal synthesis of this dipeptide The inventors thus have provided a direct microbiological method for producing the dipeptide Asp- Phe without tne need for any protecting and deprotectmg steps . The novel non-πbosomal dipeptide synthetases which, according to the present invention, can be used for the production of Asp-Phe are also indicated hereinafter as Asp-Phe dipeptide synthetases or as Asp-Phe synthetases. It is known (for instance, from P. Zuber et al . , in " Bacill us suhtilis and other Gram-positive bacteria", Sonenshe et al . (Eds.), Am. Soc. Microbiol., Washington, DC, 1993, p.897-916) that micro-organisms can produce bioactive peptides through πbosomal and non-ribosomal mechanisms. The bioactive peptides so far known to be synthesised non- πbosomally, are produced by a number of soil bacteria and fungi. These bioactive peptides can range from 2 to 48 residues, and are structurally diverse. They may show a broad spectrum of biological properties including antimicrobial, antiviral or antitumor activities, or immunosuppressive or enzyme-inhibiting activities. As such, these non-ribosomally synthesised bioactive peptides form a class of peptide secondary metabolites which has found widespread use medicine, agriculture, and biological research. Already more than 300 different residues so far have been found to be incorporated into these peptide secondary metabolites. However, until now not a single non-ribosomally formed peptide has been identified having (as a part of its peptide sequence) the dipeptide Asp-Phe it, nor has the dipeptide Asp-Phe itself been identified as a non- ribosomally synthesised product. According to the present invention Asp-Phe can now be produced non-ribosomally, and novel non- πbosomal Asp-Phe synthetases can be used for the synthesis of Asp-Phe. Hereinafter, the part of tne specification dealing with the DNA-fragments encoding the novel Asp-Phe synthetases, it will be elucidated more detail how these novel Asp-Phe synthetases can be obtained and have been made available m the context of the present invention. For better understanding of the present invention, first, however, some general background as to non-πbosomal peptide synthesis is presented. In non-πbosomal synthesis of peptides known so far generally a multiple carrier thiotemplate mechanism is involved (T. Stem et al . , J. Biol. Chem. 271, 1996, p.15428-15435) . According to this model, peptide bond formation takes place on multI -enzyme complexes which are named peptide synthetases and which comprise a sequence of ammo acid recognising modules. On the peptide synthetases a series of enzymatic reactions take place which ultimately lead to the formation of a peptide by sequential buildmg-m of ammo acids, in an order predetermined by the order of modules recognising the cognate am o acids, into the peptide. This series of enzymatic reactions includes, schematically: 1. recognition of the ammo acid substrates; 2. activation of said recognised ammo acid substrates to their ammoacyl -adenylates (that is, the ammoacyl adenosine-monophosphate ; aa-AMP) at the expense of Mg + -ATP (adenylation) ; 3. binding of the ammoacyl -adenylates the form of their more stable thioesters to the cysteamme group of the enzyme-bound 4 ' -phosphopantethemyl (4'-PP) cofactors (thiolation) . The ATP consumed m the adenylation reaction is hereby released the monophosphate form (AMP) ; 4. depending of the peptide to be synthesised non- ribosomally, the thiol -activated substrates may be modified (e.g. by epimeπsation or N-methylation) ; 5. formation of the peptide product by N to C stepwise integration of the thioesteπfled substrate ammo acids (modified, as the case may be) into the growing peptide; 6. releasing the peptide formed non-ribosomally from the template. Assuming this general scheme also to be correct for the novel non-πbosomal synthesis of Asp- Phe according to the present invention, this means that this synthesis involves the subsequent steps of d) recognition of L-Asp and L-Phe, (ii) formation of an L- Asp- and an L-Phe-acyladenylate, (m) binding thereof to the cysteamme group of the 4 ' -PP cofactor m the respective thiolation domains, (iv) formation of the Asp-Phe dipeptide by transfer of the thioester- activated carboxyl group of L-Asp to the ammo group of L-Phe, while the condensation product remains covalently attached to the multi -enzyme complex via the 4 ' -PP cofactor m the thiolation domain of the Phe- recognismg module, and (v) release of the Asp-Phe formed. According to the present invention the substrates L-Asp and L-Phe are contacted with a non- ribosomal Asp-Phe dipeptide synthetase, in the presence of an effective amount of ATP. An effective amount of ATP as meant herein is an amount of ATP which ensures that the dipeptide formation takes place at a suitable rate. Usually such rate will be at least one turn-over per minute, i.e. a turn-over number (k cat ) of 1 per minute; preferably k cat is at least 10 per minute. In order to enable an economically attractive process the ATP consumed by the peptide synthesis reaction is preferably regenerated. The contacting of the substrates L-Asp and L-Phe with the non-ribosomal Asp-Phe dipeptide synthetase may be done any suitable way; for instance - if the Asp-Phe dipeptide synthetase is present a micro-organism - L-Asp and L-Phe may be fed into the culture medium containing said microorganism. Alternatively micro-organisms may be used which are capable of overproducing L-Asp and/or L-Phe (e.g. from glucose), with separately feeding to the micro-organism of the ammo acid (L-Asp or L-Phe) which is not produced by the micro-organism. All these methods may be called m vivo methods. ATP may be regenerated m vivo m the Asp-Phe producing microorganism. The contacting of the substrates L-Asp and L-Phe with the non-ribosomal Asp-Phe dipeptide synthetase also may be done by using the synthetase its isolated form, that is by an in vi tro method. In such in vi tro methods ATP-regeneration is to be taken care of separately. This may be done by applying an ATP-regeneration system. ATP-regeneration systems are readily available to the skilled man. Protein chemical studies and recent progress m cloning and sequencing of genes encoding peptide synthetases of bacterial and fungal origin have made it clear that the known peptide synthetases have a highly conserved and ordered structure composed of so- called modules. These modules have been defined as semi -autonomous units within peptide synthetases that carry all information needed for recognition, activation, and modification of one substrate. Although the modules in principle can act independently, it is generally assumed that they have to work m concert, in a template-based mode of action to achieve peptide elongation. In general, tne modules of peptide synthetases, eacn module oemg about 1000-1400 ammo acids m length (i.e., tne modules have molecular weights m the range of 120-160 kDa), are themselves composed as a linear arrangement of conserved domains specifically representing the enzyme activities involved in substrate recognition, activation, (and, optionally, as the case may be, modification) and condensation (i.e. peptide bond formation) . Two of such distinct domains, the adenylation and thiolation domains (A-domain and T-domam) , together form the smallest part of a module that retains all catalytic activities for specific activation and covalent binding of the ammo acid substrate. Stachelhaus et al . have designated this core fragment of the modules as a "minimal module" (T. Stachelhaus et al . , J. Biol. Chem. 270, 1995, p.6163-6169) . The term "minimal module" as used here therefore, according to said definition, refers to such combined core fragment of the modules, consisting of an adenylation domain and a thiolation domain. Some highly conserved core motifs of adenylation and thiolation domains, as known to exist in peptide synthetases, are listed m table 1, together with some highly conserved core motifs of condensation and thioesterase domains (which will be addressed in more detail m later parts of this specification) . The so-called "adenylation domain" (A- domain, about 550 ammo acids) is an essential region of each module. The A-domain has been shown to bear the substrate-recognition and ATP-b dmg sites and is therefore solely responsible for activation of the recognised amino acid as its acyl adenylate through ATP hydrolysis (T. Stachelhaus et al . , J. Biol. Chem. 270, 1995, p.6163-6169) . Table 1. Highly conserved core motifs of catalytic domains of known peptide synthetases Source: M. Marahiel et al . , Chem. Rev. 97, 1997, p.2651- 2673 Very recently the first 3D structure of an adenylation domain of a peptide synthetase (PheA from GrsA) has been reported (E. Conti et al . , EMBO J. 16, 1997, p.4174-4183) . This structure shows that almost all highly conserved core motifs are positioned around the active site where the substrates are bound. The mam residues involved m building the substrate- binding pocket could also be assigned; they were found to be located between core motifs A3 and A6 and were not highly conserved. The A-domam of a module is very important in determining the specificity of the module. "Specificity" of a module means that the module has a certain preference recognising one ammo acid above other ammo acids or above another ammo acid. Of course, also the concentration of each individual ammo acid present near the module may play a role. If, for instance, the concentration of a specific ammo acid is much higher than that of (most of) the other ammo acids, the requirements for specificity may be somewhat less strict . The so-called "thiolation domain" (T- domain, about 100 ammo acids; also called peptidyl carrier protein (PCP) ) is a domain located directly downstream of the adenylation domain. It forms an integral part of peptide synthetases, and is the site of 4'-PP cofactor binding and substrate acylation. Withm the T-domam, 4'-PP is covalently bound to the sidecham of an invariant seπne residue located withm the highly conserved thiolation core motif (see table 1) . If the T-domam m a module of the peptide synthetase does not carry its 4'-PP cofactor, no covalent binding of the ammoacyl substrate can take place and chain elongation will be impossible. It has been found that peptide synthetases known so far, every T-domam is converted from the inactive apo form to the active holo form by transfer of the 4'-PP moiety from Coenzyme A (CoA) to the sidecham of the above mentioned seπne residue. This post -translational priming of each T-domam is mediated by peptide synthetase specific members of a recently discovered enzyme superfamily, the 4'- phosphopantethe yl transferases . They utilise CoA as a common substrate, and appear to attain specificity through protem/protem interactions. The Asp-Phe dipeptide synthetase as used m the method of the present invention comprises two minimal modules, respectively one minimal module at its N-termmal side recognising L-Asp and another minimal module at its C- erminal side recognising L-Phe. The term "minimal module" is used m the same meaning as given thereto by Stachelhaus et al . , J. Biol. Chem. 270, 1995, p.6163-6169. Each of these minimal modules is composed of an adenylation domain (A-domam) and a thiolation domain (T-domam) . Moreover, the two minimal modules of the Asp-Phe dipeptide synthetases according to the invention are connected by a so-called condensation domain, which needs to be covalently bound to the polypeptide chain of the Phe-module, namely to the N- terminal part thereof. The condensation domain, however, does not need to be bound covalently to both minimal modules (i.e. to the modules recognising respectively L-Asp and L-Phe) because there is no requirement that these two minimal modules are located on a single polypeptide chain. The term "connected" therefore means that the condensation domain ensures that both minimal modules can operate concertedly. In known peptide synthetases the condensation domains occur as a moderately conserved region. The condensation domain (conserved region) usually consists of about 400 ammo acid residues and is known to be involved the catalysis of non- ribosomal peptide formation. One of the conserved core motifs contains the catalytically active histidme residue. See T. Stachelhaus et al . , J. Biol. Chem. 273, 1998, p.22773-22781. The Asp-Phe formed can be recovered from the reaction medium by any method available to the skilled man. It is preferred, that the condensation domain in the dipeptide synthetase is connected to both minimal modules m such way that it is also covalently bound to the module recognising L-Asp. In such case the condensation domain is not only bound covalently to the N-terminal end of the L-Phe recognising module, but also to the C-termmal end of the L-Asp recognising module, and forms part of a single polypeptide chain comprising the L-Asp and L-Phe recognising modules. A distinguishing feature of non-ribosomal peptide synthesis is the fact that the peptide formed on the template is covalently bound to the T-domam of the C-termmal module as a peptidyl- (4 ' -PP) -T-domam intermediate. Release of the peptide from this intermediate is assumed to take place either by mtermolecular attack by water, resulting net hydrolysis, or by intramolecular capture by a hydroxyl or ammo group of the peptide chain itself, giving rise to a cyclic peptide product and of the peptide synthetase the holo form. The first termination route yields a linear peptide with a free C-termmal carboxylic group (as should be the case for the Asp-Phe synthesis according to the present invention) . Because the Asp-Phe is present an intermediate form oound to the template of tne Asp-Phe dipeptide synthetase, it is advantageous to take additional measures for enhancing the release of the Asp-Phe from said template. It is therefore particularly preferred that also a releasing factor is present for the Asp-Phe formed on the dipeptide synthetase. The term "releasing factor" as used here is intended to comprise any means, whether part of the synthetase or present m combination therewith, which enhance the releasing from the synthetase of the Asp-Phe formed on the synthetase. All known bacterial and some fungal peptide synthetase modules that incorporate the last ammo acid into the growing peptide chain show a region with a thioesterase-like function. These regions of approximately 250 ammo acids are located at the C- termmal end of the am o acid recognising modules. These thioesterase-like regions are integrated regions which exhibit homology to thioesterase-like proteins, and therefore also are referred to as the thioesterase domain ( (integrated) TE-domam) . All these integrated TE-domams contain an active site serme residue, which is part of the core motif GxSxG (see table 1) . Recent work has given support for the theory that these integrated TE-domams are involved the termination of the chain elongation and the product release. For instance, deletion of the complete TE- domam from the surfactm synthase led to a 97% reduction of the m vivo surfactm production (Schneider A., et al . , Arch. Microbiol . , 169, 1998, p.404-410) . Furthermore, it has been shown that replacing the integrated TE-domam from the C- erminus of module 7 of the surfactm synthase to the C-termmal ends of modules 4 and 5, resulted the formation of the corresponding lipotetra- and pentapeptide . Also m this study the removal of the integrated TE-domam led to an almost complete reduction of peptide synthesis (Ferra F de, et al . , J. Biol. Chem., 272, 1997, p.25304-25309) . In a particularly preferred embodiment of the invention the releasing factor therefore is a protein which shows thioesterase-like functions and forms an integrated domain of the dipeptide synthetase at the C- erminus thereof. In addition it is preferred that the Asp- Phe dipeptide synthetase, prior to the production of Asp-Phe, has undergone optimisation for its function by using one or more post-translational modifying activities. This is useful for achieving the most efficient non-ribosomal synthesis of Asp-Phe. The term "post-translational modifying activities" for efficient non-ribosomal synthesis of Asp-Phe as used here is intended to comprise any activities which modify the dipeptide synthetase after its formation thereby positively affecting its Asp-Phe synthesismg function. In particular, m the production of Asp-Phe according to the present invention the post- translational modifying activity used is a 4 ' - phosphopantethemyl (4 ' -PP) transferase. The 4 ' -PP transferase provides for effective conversion of the apo- to holo-enzyme of the peptide synthetase and by loading the 4 ' -PP cofactor to the ser e side-chains the core motif of the T-domams, and thereby increases the yield of Asp-Phe m the production thereof. Effective conversion of apo- to holo-enzyme is provided if in each of both T-domains of the Asp-Phe dipeptide synthetase at least 10% of the apo-enzyme is converted to the holo-form. It is particularly preferred that the production of Asp-Phe according to the invention also a non- integrated protein with thioesterase Type-II-like activity is present together with the dipeptide synthetase. As meant herein proteins having thioesterase Type-II-like activity are proteins with strong sequence similarities to type-II fatty acid thioesterases of vertebrate origin. Such non-integrated protein with thioesterase Type-II-like activity is different from the integrated thioesterase (TE-domam) . Recent work (Schneider et al . , Arch. Microbiol . (1998), 169, 404-410) has shown that deletion of a gene encoding such non-integrated protein with thioesterase Type-II-like activity from the surfactm synthase operon leads to an 84% reduction of peptide production. It is suggested that the non- integrated protein with thioesterase Type-II-like activity enhances production of non-ribosomal peptides, possibly by reactivation through liberation of mischarged modules that are blocked with an incorrect ammoacyl group or an undesired acyl group at the 4'-PP cofactor. The genes coding for the non- integrated proteins with thioesterase Type-II-like activity can be positioned at the 5'- or 3 ' -end of the peptide synthetase encoding operon. These proteins have molecular masses of 25-29 kDa, are about 220-340 am o acid residues m length, and carry the sequence GxSxG which is presumed to form the active site. It is noticed that in almost all of the prokaryotic peptide synthetase coding operons known so far, such distinct genes have been detected. In the production of Asp-Phe according to the present invention the Asp-Phe dipeptide synthetase is preferably present living cell-material of a micro-organism, and glucose, L-Asp and/or L-Phe are being fed to said fermentor, and the Asp-Phe formed is then recovered. As used in this context the term "glucose" is intended to cover glucose and any otner energy source necessary for the regeneration of ATP the living cell material and for the mamtamance energy required for said living cell material. The glucose (or other energy source) , moreover, is used as starting material for the production of any L-Asp and/or L-Phe to be produced m the living cell the course of the process of the invention. The skilled man, of course, will be aware that the feeding of glucose, L-Asp and/or L-Phe is to be done under appropriate conditions of temperature and pH, including as required the presence of an appropriate nitrogen source, salts, trace elements, and other organic growth factors as vitamins and ammo acids, etc. to the fermentor or other type of (enzyme) reactor which is used for the production of Asp-Phe. The Asp-Phe formed is recovered. Such recovery may take place during the process or at the end thereof. The living cell -material may be present m any appropriate form as available to the skilled man. For instance, whole cells may be used as such or m immobilised form. The micro-organism may be any kind of micro-organism wherein the Asp-Phe dipeptide syntnetases according to the invention can stably be expressed. Suitable micro-organisms are, for instance, micro-organisms which (a) are producing peptides via non-ribosomal synthesis, for instance, bacteria as Strepto yces species, Bacillus species, Actmomyces species, Micrococcus species, Nocardia species, or fungal species as Tolypocladiu species, Fusaπ um species, Pem cillium species, Aspergillus species, and Cochliobolus species; or (b) are capable of producing ammo acids, m particular L-Asp and/or L-Phe, preferably on industrial scale, for instance, Escheπchia species, e.g. E. coli , and Corynebacteπum species, e.g. C. glutamicum. The micro-organisms may be grown, under conditions which can easily be found by the skilled man, m a fermentor, and production of the Asp-Phe then can be carried out m the same or another fermentor. As meant herein the fermentor may be any type of fermentor or other types of (enzyme) reactor known to the skilled man. In the method for the production of Asp-Phe according to the present invention it is preferred that the micro-organism is first grown m a fermentor to reach a predetermined cell density before the expression of the Asp-Phe dipeptide synthetase is switched on and feeding of the glucose, L-Asp and/or L- Phe for the synthesis of the Asp-Phe dipeptide is started. The skilled man can easily determine the growth of the micro-organism, e.g. by measuring its optical density (O.D.), and find the most appropriate level of cell density. To prevent any negative effect on the growth of the micro-organism, growth phase and Asp-Phe synthetase productio phase are preferably uncoupled. Such uncoupling can be achieved by expressing the gene for the Asp-Phe synthetase from an mducible, tightly regulable, promoter. The expression of the Asp-Phe dipeptide synthetase is preferably switched on by addition of a specific chemical component (mducer) or by changing the physical conditions, e.g. the temperature, pH or dissolved oxygen pressure, after a predetermined level of cell density has been reached. The expression is assumed to be switched-on as compared to the non- induced state, if the expression level of the Asp-Phe dipeptide synthetase is raised at least by a factor of 10. Then also the feeding of substrates, etc. m amounts as required, is started, and production of Asp-Phe starts. Most preferably the micro-organism is an L- phenylalanme producing micro-organism and, apart from required amounts of salts and trace elements etc., only glucose and L-Asp are being fed. L-Phe producing microorganisms are well-known. For instance, E. coli and Corynebacterium species are being used for L-Phe production. By expressing an Asp-Phe dipeptide synthetase m such micro-organisms availability of L- Phe in the micro-organism is provided for, and only glucose, an appropriate nitrogen source, organic growth factors, salts and trace elements, etc., as well as L- Asp, should be supplied as required. In particular the micro-organism used is an Escherichia or a Bacillus species. Best results are obtained if the micro- organism used is a strain with reduced protease activity for Asp-Phe or lacking such activity towards Asp-Phe. By using such strains degradation of Asp-Phe formed is prevented. Any suitable strain which lacks protease activity (either naturally or because the activity of the protease has been lowered substantially or has been removed completely) may be used. Moreover, if the synthesis of Asp-Phe occurs m a micro-organism, also additional measures can be taken for improving the permeation of the Asp- Phe formed into the reaction medium outside the microorganism and recovering tne Asp-Phe therefrom after separation of the micro-organism from the reaction medium. Similarly, also additional measures may be taken for improving the uptake of glucose and/or L-Asp and/or L-Phe. In an even more preferred embodiment of the invention the micro-organism used also contains a suitable export system for Asp-Phe formed and/or one or more suitable uptake system(s) for glucose and/or L-Asp and/or L-Phe. Using a suitable export system will ensure achieving more efficient secretion of the Asp- Phe formed. The secretion meant here is the secretion of Asp-Phe formed m the micro-organism into the extracellular environment . Efficient secretion of Asp- Phe is important for improving the recovery yield of Asp-Phe and for maintaining the activity of the Asp-Phe dipeptide synthetase at a suitable level as well as for preventing intracellular degradation of Asp-Phe. Moreover, the down-stream processing for Asp-Phe secreted is more easy. Similarly, the presence of suitable uptake system(s) will improve the coupling efficiency to Asp- Phe. In the foregoing paragraphs in vivo non- ribosomal synthesis methods for Asp-Phe have been described. They all are characterised m that living cell material is used and ATP regeneration takes place in this cell material. The present invention can also oe carried out m vi tro . As used herein, m vi tro systems are characterised that the Asp-Phe dipeptide synthetase is not present living cell material; it may, however, be present any other environment, for instance m permeabilised cells, cell-free extract, or as an isolated dipeptide synthetase. In such case regeneration of ATP does not take place m living cell material used for the synthesis of Asp-Phe, and special measures for supply of ATP m an effective amount have to be taken. In a preferred embodiment of the invention, the production of Asp-Phe is carried out m vi tro m an enzyme reactor, while ATP is supplied, and L-Asp and/or L-Phe are being fed, and the Asp-Phe formed is recovered. In order to improve the economic feas oility of the process it is particularly preferred to increase the yield of Asp-Phe per mole of ATP supplied for the synthesis of Asp-Phe. This can be achieved by in situ ATP regeneration from the AMP formed out of the ATP in the consecutive adenylation and thiolation reactions. Therefore, in a preferred mode of the invention, the supply of ATP is provided part by an m situ ATP-regenerating system. Various ATP regenerating systems (which the literature are also being referred to as ATP generating systems) are known to the skilled man. As ATP regenerating systems both whole cell systems (e.g. yeast glycolysis systems) or isolated ATP regenerating enzymes, for instance adenylate kinase combined with acetate kinase, may be used. A very elegant ATP regeneration system has been described by T. Fu]io et al . (Biosci., Biotechnol . , Biochem. 61, 1997, p.840- 845) . They have shown the use of permeabilised Corynebacterium a momagenes cells for regeneration of ATP from the corresponding monophosphate (AMP) coupled to an ATP-requirmg reaction m permeabilised E. coll cells. In this elegant way (cheap) glucose can be supplied as an energy source instead of most cf the ATP. Therefore, the ATP-regenerating system is preferably present in a permeabilised micro-organism. This permeabilised micro-organism present in the (enzyme) reactor used ensures that an effective amount of adenosine- triphosphate (ATP) will always be present and available during the Asp-Phe production according to the present invention. DNA FRAGMENTS ENCODING AN Asp-Phe DIPEPTIDE SYNTHETASE, ETC. The present invention also relates to novel DNA fragments encoding an Asp-Phe dipeptide synthetase. These novel DNA fragments or combination of DNA fragments code for a non-ribosomal Asp-Phe dipeptide synthetase, which synthetase comprises two minimal modules connected by one condensation domain wherein the N-termmal module of these modules is recognising L-aspartic acid and the C-termmal module of these modules is recognising L-phenylalamne and is covalently bound at its N-termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' - phosphopantetheinyl cofactor containing thiolation domain. The term "DNA- fragment or combination of DNA fragments" as used herein is understood to have its broadest possible meaning. The term first of all relates to the composite biological material (on one or more DNA-fragments) as mentioned herein-above and coding for the minimal modules for Asp and Phe the correct order and for the condensation domain, each coding sequence being surrounded by any transcription and translation control sequences (e.g. promoters, transcription terminators) and the like which may be suitable for the expression of the Asp-Phe dipeptide synthesis g activity. The control sequences may be homologous or heterologous, and the promoter (s) present m the DNA may be constitutive or ducible. The term "DNA-fragment " as used herein is further understood to code, m addition to coding for the Asp and Phe minimal modules and the condensation domain, for the activities of the other domains, e.g. TE-domams. Furthermore, these fragments may code"'*-?ό * activities which are not located on the Asp-Phe dipeptide synthetase polypeptide itself, sucn as non- mtegrated thioesterase Type-II-like proteins, and other activities co-operating concertedly with the Asp and Phe minimal modules . The term "DNA- ragment " as used herein is also understood to comprise gene structures comprising DNA fragments as described herein-above. More precisely, a gene structure is to be understood as being a gene and any other nucleotide sequence which carries the DNA-fragments according to the invention. Appropriate nucleotide sequences can, for example, be plasmids, vectors, chromosomes, or phages . The gene structures may exist either as (part of) an autonomously replicating vector in single or multicopy situation, or integrated into the chromosome m single or multicopy situation. The gene structure is also to be understood as being a combination of the above-mentioned gene carriers, such as vectors, chromosomes and phages, on which the DNA- fragments according to the invention are distributed. For example, the Asp-Phe dipeptide synthetase encoding DNA- fragment can be introduced into the cell on a vector and the non-integrated thioesterase Type-II-like protein encoding DNA-fragment can be inserted into the chromosome. In addition, a further DNA-fragment can, for example, be introduced into the cell using a phage . These examples are not intended to exclude other combinations of DNA-fragment distributions from the invention. The DNA-fragments according to the invention may be introduced into the micro-organism at a sufficiently high copy number, for instance of up to 50 copies. A detailed discussion of the Asp-Phe dipeptide synthetase and the two minimal modules comprised therein already has been given m the preceding parts of this patent application. The construction of the DNA fragments according to the present invention is not self-evident, although terms like "module", "domains", etc. mislead gly might suggest that much is known about the functional boundaries thereof. Such detailed information which would enable rational design of (mutant, non- natural) peptide synthetases, however, is not yet available. Nevertheless, recently various techniques for construction of mutant peptide synthetases have been described literature. Methods for construction of mutant peptide synthetases described m literature are: De Ferra et al . (J. Biol. Chem., 272, 1997, p.25304 -25309) have described a method for producing truncated peptides of a predicted sequence by replacement of the integral TE-domam of the surfactm synthetase from the C-termmal module to the C-terminal end of different internal modules. This technique alone, however, is not suitable for the construction of an Asp- Phe dipeptide synthetase module because the natural order of two consecutive Asp and Phe modules is not known to exist m nature (neither at the N- or C- termmal end of any natural synthetase, nor as an internal sequence of any naturally occurring synthetase) . Another in vi vo technique described for engineering peptide synthetases is the so-called programmed alteration with the primary structure of a peptide product Basis for this method is the replacement of one module by anotner on the genetic level . This technique has been described m general by A. Schneider et al . , Mol. Gen. Genet., 257, 1998, p.308-318) . In this way ammo acid-activatmg minimal modules could be exchanged successfully m vivo between multi -modular peptide synthetases of heterologous origin, and micro-organisms could be obtained which indeed produce non-ribosomal peptides with a different primary structure from the peptides produced without such alteration. If this technique would be applied for the construction of an Asp-Phe dipeptide synthetase, in principle two options would be available, each starting from a dipeptide synthetase, namely having a sequence of two modules comprising either (I) Asp and XXX, the latter representing any other ammo acid than Phe, or (ii) YYY and Phe, the former representing any other ammo acid than Asp. In those dipeptide synthetases the DNA coding for the XXX module should be replaced by DNA coding for the Phe module, or the DNA coding for the YYY module by DNA coding for the Asp module. Other methods for construction of peptide synthetases have been described in EP-A-0637630. In said patent application a method s suggested whereby, next to alteration of the substrate specificity by substitution of (part of) modules, also modules can be deleted or inserted into the synthetase chain. "Specificity" of a module means that the module has a certain preference m recognising one ammo acid above other ammo acids or above another ammo acid. It is a distinctive feature of the above peptidase engineering methods that homologous recombination events are used to bring about the desired changes m the genomic DNA m the native peptide producing micro-organism. Because the homologous recombination events take place m the native peptide producing micro-organism, these methods would have the advantage that ail the native nost enzymes and relevant regulatory elements are present principle . However, homologous recombination through use of the native non-ribosomal peptide producer suffers from several severe drawbacks . The most serious of these drawbacks is that it is often tedious and technically difficult, especially when applied to slow- growing micro-organisms with poorly developed transformation systems or which are lacking m other genetic tools. Other drawbacks are that the native non- ribosomal peptide producing micro-organisms often do not have a history of safe use on industrial scale, are no production organisms for L-Phe and/or L-Asp, and have unknown fermentation characteristics. Moreover, all these methods result in cells having only a single copy of the DNA-fragment coding for the desired peptide synthetase. Therefore, none of these in vivo engineering methods are suitable for the preparation of the novel Asp-Phe dipeptide synthetase according to the invention and use thereof for the industrial production of Asp- Phe. The present inventors now have found that the Asp-Phe dipeptide synthetase can be readily obtained by use of m vi tro engineering techniques. So far no in vi tro engineering techniques for the construction of peptide synthetases have been described. Detailed protocols for the construction of Asp-Phe dipeptide synthetases according to the invention can be found m the experimental part of this application. The Asp-Phe dipeptide synthetase encoding DNA-fragment can be constructed in vi tro from an Asp-XXX or YYY-Phe (with XXX and YYY having the meaning as described above) dipeptide synthetase encoding DNA- fragment (or partial sequences for such synthetase encoding fragments as occurring in a naturally existing peptide synthetase) . This was accomplished starting from an Asp-Leu (Leu = leucine) dimodular peptide synthetase encoding DNA-fragment which was obtained from the Bacillus subtilis ATCC 21332 surfactm synthetase A gene (srfA-B) by PCR method. The Leu minimal module encoding DNA-fragment thereof then was replaced by a DNA-fragment (obtained by PCR method) from the Bacillus brevis ATCC 8185 tyrocidme A synthetase gene coding for a Phe minimal module ( tycA) . Then an integral TE-domam was added to the C-termmal end of the Asp-Phe encoding DNA- fragment by replacement of the thiolation domain of the Phe module by a PCR-fragment coding for the srfA-C thiolation and TE domain. This construction was done m such a way that the DNA encoding the additional TE domain was fused in-frame with the gene encoding the Asp-Phe synthetase. As a result the TE-domam forms an integrated part of the Asp-Phe synthetase produced. In the experimental part of this application this TE-domam containing Asp-Phe synthetase will be referred to as Asp-Phe-TE. After the construction, the encoding DNA- fragments were introduced into a suitable host microorganism. Suitable host micro-organisms are, for instance, E. coli and Bacillus species. After cultivation of these micro-organisms under inducing conditions, cells were lysed and the synthetases produced were purified by IMAC (Immobilised Metal Affinity Chromatography) . The purified enzyme preparations were used for different experiments to prove the formation of Asp-Phe. Preferred DNA fragments The following preferred aspects of the DNA fragments encoding the Asp-Phe dipeptide synthetase according to the invention closely correspond to the aspects discussed in the previous parts of this specification regarding the preferred methods for the production of Asp-Phe. For the DNA fragments encoding the Asp-Phe dipeptide synthetase according to the invention it is especially preferred that the condensation domain m the encoded dipeptide synthetase is connected to both minimal modules m such way that it is also covalently bound to the module recognising L-aspartic acid. In particular it is preferred that the DNA fragment or combination of DNA fragments encoding the dipeptide synthetase also code for a releasing factor for the Asp-Phe formed on that dipeptide synthetase. The term "releasing factor" is used in the same meaning as it has been used m the previous part of the specification. In a more particularly preferred embodiment of the present invention, the DNA fragment or combination of DNA fragments encoding the Asp-Phe dipeptide synthetase is/are also coding for a protein which shows thioesterase-like functions and forms an integrated domain of the dipeptide synthetase at the C- termmus thereof. For an explanation of the terms "integrated domain" etc., reference is made to earlier parts of the present application. In addition, the synthetase encoding DNA fragment or combination of DNA fragments preferably also express (es) one or more post-translational modifying activities for efficient non-ribosomal synthesis of Asp-Phe on the synthetase. The terms "post-translational modifying activities", etc. are used in the same meaning as they have been used m the previous part of the specification. In particular, the post-translational modifying activity expressed by the DNA fragment or combination of DNA fragments is a 4 ' - phosphopantethemyl (4 ' -PP) transferase activity. The formation of this activity provides for effective conversion of apo- to holo-enzyme. Effective conversion of apo- to holo-enzyme, etc. already has been explained m the previous part of the specification. It is particularly preferred that the DNA fragment or combination of DNA fragments also code(s) for a non- integrated protein with thioesterase Type-II- like activity. The term "non-mtegrated protein with thioesterase Type-II-like activity" is used the same meaning as it has been used the previous part of the specification. Micro-organisms The invention further relates to microorganisms containing a DNA fragment or combination of DNA fragments according to the invention, and in particular to such micro-organisms which are capable of producing L-Asp and/or L-Phe. In particular, the microorganism is an Escherichia coli or Bacillus species. Asp-Phe dipeptide synthetases The present invention finally also relates to novel Asp-Phe dipeptide synthetases. The terms and expressions used hereinafter with respect to the Asp-Phe dipeptide synthetase all have the same meaning as explained herein-above. The non-ribosomal Asp-Phe dipeptide synthetases according to tne present invention are characterised m that they comprise two minimal modules connected by one condensation domain wherein the N- terminal module of these modules is recognising L- aspartic acid and the C-termmal module of these modules is recognising L-phenylalanme and is covalently bound at its N-termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' - phosphopantethemyl cofactor containing thiolation domain. In particular, the condensation domain in the dipeptide synthetases is connected to both minimal modules m such way that it is also covalently bound to the module recognising L-aspartic acid. Preferably, the Asp-Phe dipeptide synthetase also comprises a releasing factor for the Asp-Phe formed on that dipeptide synthetase. Most preferably, the releasing factor is a protein which shows thioesterase-like functions and forms an integrated domain of the dipeptide synthetase at its C-termmus. The invention hereinafter now will be clarified further m the experimental part, but will in no way be restricted to the experiments shown. Ammo acids used all were enantiomerically pure L-ammo acids. EXPERIMENTAL PART General procedures Standard molecular cloning techniques such as DNA isolation, gel electrophoresis, enzymatic restriction modifications of nucleic acids, E. coli transformation etc., were performed as described cy Sambrook et al . , 1989, "Molecular Cloning: a laboratory manual", Cold Spring Harbor Laboratories, Cold Spring Harbor, New York and Inms et al . , 1990, "PCR protocols, a guide to methods and applications" Academic Press, San Diego. Synthetic oligo deoxynucleotides were obtained from MWG-Biotech AG, D- Ebersberg. DNA sequence analyses were performed on an Applied Biosystems ABI 310 genetic analyzer, according to supplier's instructions. Sequencing reactions were carried out by the chain termination method with dye- labelled dideoxy terminators from the PRISM ready Reaction DyeDeoxy Terminator cycle sequencing kit with AmpliTaq FS polymerase (Applied Biosystems) . Construction of plasmid pasp-leu-H±Sβ A 4934 bp fragment comprising regions from the srfB locus from chromosomal Bacillus subtilis ATCC 21332 DNA was amplified (PCR) using the following primers: 5' TAA GCA TGC TGC TTT CAT CTG CAG AAA C (5' asp-leu- Sphl - srfB2 ) , and 3' AAT GGA TCC TTC GGC ACG CTC TAC (3' asp-leu-Ba-nHI- srfB3) . Correct size of the amplified fragment was confirmed by agarose gel electrophoresis . The fragment (20 μg) was digested with 1 unit of the enzymes BairiHI / Sphl (37°C, 16 h) to generate terminal restriction sites. Plasmid pQE70 (provided by Qiagen, D-Hilden) (10 μg) was digested with the same enzymes and suDsequently incubated for 1 hour with 1 unit Alkaline Phosphatase (37°C) . Complete digestion was confirmed by transforming 1 μL of the linearised plasmid DNA into competent cells of E. coli XL1 blue. The two fragments were subsequently ligated in a ligation reaction (10 μL) in a vector/insert ratio of 1:3 with 1 unit of T4-DNA- ligase enzyme (16°C, 16 h) . 1 μL of the ligation mixture was used to transform 40 μL competent cells of E. coli XL1 blue (Stratagene, D- Heidelberg) by electroporation. The transformants were selected on 2x YT agar plates containing Ampicillin (100 μg/mL) . Analysis of 48 transformants resistant to ampicillin revealed that 4 of them had inserted a ca . 5000 bp fragment. Correct insertion was confirmed using restriction enzyme digestion analysis and terminal sequencing of the insert. A correct clone designated pasp-leu-Hiss was used for further investigations. Construction of plasmid pasp-phe-His 6 Plasmid pasp-phe-His 6 was constructed from plasmid pasp-leu-His 6 as follows. A 1894 bp chromosomal DNA-fragment from Bacillus brevis ATCC 8185 DNA was amplified (PCR) using the following primers: 5' ATT TGG TCA CCA ATC TCA TCG ACA A (5' BstEII-TycA- NLID) , and 5' ATA GGA TCC TGT ATT CGT AAA GTT TTT C (3'-PheAT- Correct size of the fragment was confirmed using agarose gel electrophoresis . The fragment was digested with 1 unit of enzyme BairiHI and incubated at 30°C for 4 hours. Subsequently 1 unit of enzyme Bst II was added and incubated for another 4 hours at 60 °C. Plasmid pasp-leu-Hisς was digested m the same way and subsequently incubated for 1 hour with 1 unit of Alkaline phosphatase. The vector portion (ca. 6,5 kb) was separated from other DNA fragments by agarose gel electrophoresis and repuπfied. Complete digestion was confirmed as before with linearised pasp- leu-Hιs s . The two fragments were ligated in a equimolar ratio for 5 hours at 16°C using 1 unit of T4-lιgase enzyme. 1 μL of the ligation mixture was used for electroporation of E. coli XL1 blue competent cells. Transformants were selected on 2x YT agar containing Ampicillin (100 μg/mL) . Analysis of transformants revealed that 1 out of 90 clones had inserted a fragment of ca. 2000 bp . Correct insertion was confirmed using restriction enzyme digestion analysis and terminal sequencing of the insert. The correct clone was designated pasp-phe- His 6 . In contrast to the peptide synthetase encoding gene on plasmid pasp-leu -His e , the peptide synthetase encoding gene on plasmid pasp-phe-Hιs 6 is a hybrid gene obtained by exchanging the DNA-fragment coding for the second (Leu) minimal module (A- and T-domam) , for a DNA-fragment coding for a Phe minimal module. Construction of plasmid pasp-p e-TE-Jϊis 6 Plasmid pasp-phe-TE-Hιs 6 was constructed from plasmid pasp -phe- His 6 . A 910 bp chromosomal DNA-fragment from Bacillus subtilis ATCC 21332 DNA was amplified (PCR) using the following primers : 5' ATA ATC GAT AAT CGC ACA AAT ATG GTC (5' TE-srfCl- Clal) and 3' ATA AGA TCT AAC AAC CGT TAC GGT TTG TGT (3' mt TΞ- srfCl -Bgll l ) . Correct size of the fragment was confirmed using agarose gel electrophoresis . The fragment was digested with 1 unit of enzyme Clal for 4 hours at 37°C, before adjusting buffer conditions and digesting with 1 unit of enzyme Bgll l (4 hours, 37°C) . Plasmid pas -phe- His 6 was digested with enzyme Clal (4 h, 37°C) and subsequently with Ba-riHI (4 h, 37°C) before the linearised plasmid was incubated for one hour with 1 unit of Alkaline phosphatase. The vector portion (ca. 8 kb) was separated from other DNA-fragments by agarose gel electrophoresis and repuπfied. Control of complete digestion, ligation, electroporation and selection of transformants was established as described before. Two of the analysed transformants were shown to contain the desired DNA-fragment. Correct insertion of the 900 bp fragment was confirmed by restriction enzyme analysis and terminal sequencing of the insert. A correct clone was designated pasp-phe-TE- Hi s s . In contrast to the peptide synthetase encoding gene on plasmid pasp-phe-Hιs 6 , the peptide synthetase encoding gene on plasmid pasp-phe-TE-Hιs 6 contains a second fusion site located between the DNA coding for the adenylation domain and thiolation domain of the second (Phe) minimal module. The C-termmal T-TE domains resemble the native C-terminus of the Surfactm synthetase srfC . Expression of the peptide synthetases asp-leu -His 6 , asp- phe -Hls s and asp-phe -TE-His 6 1 μL of each constructed plasmid were transformed m E. coli BL21/pgrsp competent cells. Strain BL21 1DE3 was obtained from Stratagene, D-Heidelberg. Plasmid pgsp, which is cased on plasmid pREP4 (obtained from Qiagen, D-Hilden) , contains the gsp gene (the 4 ' -PP transferase gene from the Gramicidin S-biosyntnesis operon from Bacillus brevis ATCC 9999) under control of the T7 promoter. Transformants were selected on 2x YT agar plates containing Ampicillin (100 μg/mL ) and Kanamyc (25 μg/mL) . Several colonies were used to inoculate 4 mL of 2x YT liquid medium (containing m addition 10 mM MgCl 2 ) and incubated at 37°C for 16 hours. These 4 mL cultures were subsequently used to inoculate 400 mL of the same medium. Cells were grown at 30°C m a waterbath shaker (250 rpm) . After 3-4 hours the cells reached an optical density of 0,7 (OD 6 -o nm ) and were induced by the addition of 200 μM IPTG. Cells were incubated for an additional 1,5 hours before being harvested. Expression of recombinant proteins was confirmed by SDS-PAGE comparing protein samples taken at the time of induction and 1,5 hours later. In crude cell extracts from BL2 l/pgsp /pasp - leu-HiSς and BL2l/pgrsp/pasp-p e-His s expression of an mducible protein of ca. 180 kDa was confirmed. From crude cell extracts of BL21 /pgsp/ pasp -phe-TE- i s 6 expression of an mducible protein of ca. 200 kDa could be shown . From cultures expressing the correct recombinant proteins glycerol stocks were prepared and stored at -80°C. Purification of the recombinant proteins Asp-Leu-Hisβ/ Asp-Phe-Hisg and Asp-Phe-TE-His s 800 mL cultures of BL21/ pgsp/pasp-leu-Hιs 6 , BL21/ pgsp /pasp -phe- Hιs 6 and BL21 /pgsp /pasp -phe-TE- His s treated as described in "Expression of the peptide synthetases ..." were centrifuged at 5000 rpm for 5 minutes and resuspended m 30 mL/L culture of buffer A (50 mM HEPES, 300 mM NaCl , pH 8,0) . Cell suspensions were used directly or were stored at -20 °C till usage. Cell lysis was established using two French press passages at a working pressure of 12000 psi. Directly after cell lysis PMSF was added to a final concentration of 1 mM. After centrifugation of the cell lysates at 10000 rpm for 30 minutes, the supernatant was combined with 1% (v/v) buffer B (50 mM HEPES, 300 mM NaCl , 250 mM Imidazol, pH 8,0). Protein solutions were applied on a Nι + -NTA-agarose column (Qiagen, D-Hilden) previously equilibrated with 1% (v/v) buffer B. Flow rate was 0,75 mL/mm. After the non-Hιs s - tagged proteins had passed through the column, it was washed with 1% buffer B for another 10 mm before a linear gradient was applied (30 mm to 30% B, an additional 10 mm to 100% B) . All three proteins eluted at a concentration of about 5% buffer B (15 mM Imidazol) and were collected as 2 mL fractions. Fractions containing the recombinant proteins were detected using the Bradford reagent, by the absorption at 595 nm. These fractions were pooled and dialysed against a buffer containing 50 mM HEPES, 100 mM NaCl, 10 mM MgCl 2 and 5 mM DTE for 16 hours. After dialysation protein concentrations were again determined. Till further usage proteins were stored at - 20°C after addition of glycerol to 10% (v/v) . From 1 L culture approximately 5 mg of each pure recombinant protein could be obtained. Grade of purification was estimated to be 95% by SDS-PAGE. Analysis of enzymatic activity ATP -PP X - exchange reaction : Specificity of ammo acid activation was determined indirectly by incorporation of labelled 32 PP X into ATP during reverse reaction (Lee, S.G. & Lipmann, F.; Tyrocidme synthetase system; Methods Enzymol . 43, 1975, p.585-602) . For this purpose 20 pmol of each enzyme was incubated with 1 mM ammo acid, 1 mM ATP, 0,1 mM PP X , 50 mM HEPES, 100 mM NaCl and 10 mM MgCl 2 and 2 mCi 32 PP 1 at 37°C a total volume of 100 μL . Reactions were quenched after 10 mm by adding 500 μL of a solution containing 100 mM NaPP x , 560 mM perchloric acid and 1,2% (w/v) active charcoal. The mixture was centrifuged at 13000 rpm for 1 mm. The pellet was washed and resuspended twice with 1 mL H 2 0. Incorporation of labelled ATP (adsorbed to the charcoal) was detected by measuring radioactivity of the precipitate. Asp-Leu-His 6 was shown to activate Asp and Leu exclusively. K values for Asp and ATP were determined to be 3,5 mM and 0 , 9 mM respectively. , values for Leu and ATP were detected to be 0,3 mM and 0,6 mM, respectively. Asp-Phe- His 6 and Asp-Phe-TE-Hιs 6 were shown to activate both Phe and Asp. The , value for Phe was determined to be about 50 μM. The ammo acid activation patterns of Asp- Phe-Hιs s and Asp-Phe-TE-Hαs s were found to be identical. Covalent binding of ammo acids to the Asp -Phe dipep tide syn thetase : Quantity of holo-enzyme formation was determined by measuring the amount of labelled Asp, Leu and Phe, that could be covalently oound to one equivalent of the purified proteins Asp-Leu-Hιs s , Asp- Rhe-His s and Asp-Phe-TE-His s . (Lee, S.G ; see aoovej . 50 pmol of each enzyme was incubated with 2 mM ATP, 50 mM HEPES, 100 mM NaCl, 10 mM MgCl 2 and 100 pmol of 1 C labelled ammo acid (Asp and Leu or Asp and Phe respectively) at 37°C. After 30 mm the reaction was quenched by the addition of 1 mL of 10% TCA and 5 mg/mL BSA and subsequently stored at 0°C for another 30 mm. The protein precipitates were collected by centrifugation (30 mm at 13000 rpm) and washed twice with 10% TCA. The washed precipitates were dissolved in 50% performic acid and used to measure incorporation of labelled ammo acid. Asp-Leu-H s s could be labelled with Asp and Leu to a degree of approximately 20-25%. Asp-Phe-His 6 and Asp-Phe-TE-His 6 could be labelled with Asp to a degree of only 10-15%. The incorporation of Phe reached a level of approximately 50%. Jndirect proof of forma tion of the dipeptides Asp-Leu and Asp-Phe : Covalent binding of the constituent ammo acids to the dipeptide synthetases was shown by kinetic experiments using radioactively labelled Asp. In a first assay 0,85 μM Asp-Leu-His 6 were incubated with 2 mM ATP, 2,8 μM Asp ( 14 C, 56 nCi) in a buffer (50 mM HEPES, 10 mM MgCl 2 , 100 mM NaCl, pH 8,0) at 37 °C. At regular time intervals samples were taken and treated as indicated m the previous paragraphs; radioactivity (of Asp covalently bound to the enzyme) was measured m each sample. After 4 minutes the amount of incorporated labelled Asp started levellmg-off to reach a maximum a few minutes later if no second ammo acid was added. If, however, 1,5 μM of Leu were added after 4 minutes, a further strong, but temporary, increase of radioactivity was observed, which sharply decreased after about 5 minutes to a level below the maximum observed when no second ammo acid was added. In another experiment 0,5 μM Asp-Phe-His 6 were incubated m the same way as Asp-Leu-Hιs s before. In this case the addition of 0,1 mM Phe after about 5 minutes resulted a similar temporary increase of covalently bound radioactive Asp, as described above. However, the addition of Leu instead of Phe did not lead to such temporary increase. This clearly shows that peptide bond formation takes place on each of these peptide synthetases. Furthermore, the results show that the peptides formed on the peptide synthetases are being released therefrom. Direct proof of forma tion of the dipeptide Asp-Phe : Formation of the dipeptide Asp-Phe was proven by a series of experiments, using different analytical techniques, namely thm layer chromatography (TLC) with radio-active detection, high performance liquid chromatography and mass spectroscopy. In a first set of experiments (method to determine Asp-Phe released from the dipeptide synthetase) 100 pmol of Asp-Phe-His 6 were incubated m a total volume of 200 μl with ImM of ATP, 1 mM Phe and 1.25 μM 14 C-labeled Asp a buffer (50 mM HEPES, 20 mM MgCl 2 , pH 8) for 6 hours at 37 °C. Control experiments carried out were done omitting one of ATP, Phe or enzyme . In a further, analogous, set of experiments the same amount of said enzyme was incubated under identical conditions with 1 mM Asp and 1.25 μM R elabeled Phe (instead of Phe and 14 C-labeled Asp) . Also the appropriate blanks were carried out . To finish the reactions, 100 μl of n-butanol were added to precipitate the enzyme which then was removed. The remaining clear solutions were evaporated and replenished to a volume of 20 μl with 10 vol% of methanol in water. An appropriate volume of each of these samples was applied to a silica TLC plate, which then was developed using a solution of butanol/acetic acid/ethyl acetate/water 1:1:1:1 (v/v/v/v) as the eluent . After elution the plate was dried and an X-ray film was put on top of it for 2 to 5 days of exposure. Finally the film was processed to show spots of radioactively labeled compounds. Only if Asp, Phe, ATP and enzyme were present in the reaction mixture a spot could be detected that had the same retention factor (R f ) as Asp-Phe. In a second set of experiments (method to determine dipeptide synthetase oound Asp-Phe) 1 nmol of Asp-Phe-Hιs 6 were incubated a total volume of 600 μl with ImM of ATP, 1 mM Asp and 1 μM 14 C-labeled Phe m a buffer (50 mM HEPES, 20 mM MgCl 2 , pH 8) at 37 °C. Reactions were stopped after 10, 20 and 30 minutes, respectively, by the addition of 300 μl of 20% (weight) aqueous tπchloroacetic acid (TCA) and the mixtures were cooled to 4 °C. All further steps were carried out at 4 °C. The precipitated protein was collected by centrifugation, washed once with 500 μl of 10% aqueous TCA, then with 1 ml of a 3 : 1 (v/v) mixture of diethyl ether and ethanol, and finally with 1 ml of diethyl ether. Next the pellet was dried for 15 minutes at 37 °C, before being resuspended in 200 μl of 100 mM of an aqueous solution of potassium hydroxide under vigorous shaking. The mixture then was treated for 0.5 hours at 70 °C to release any thioester bound dipeptide from the enzyme. To separate the released dipeptide form the protein, the solution was replenished to 1 ml with methanol. This solution was centrifuged for 30 minutes at 4 °C and the supernatant was evaporated in vacuo for 3 hours at room temperature. The obtained pellet was finally resuspended in 25 μl of 10% aqueous methanol . Analysis then was done by TLC w th radioactive detection as descπoed the previous set of experiments. A clear spot of the dipeptide Asp-Phe could be observed m each of the three samples. The intensity of these spots increased significantly for the samples taken at 10, 20 and 30 minutes, respectively. This shows that formation of the dipeptide Asp-Phe really takes place on the dipeptide synthetase. In a final experiment formation of Asp-Phe was also confirmed by comparing the HPLC retention times and mass-spectra of the dipeptide formed. In a total volume of 200 μl 50 pmol of Asp-Phe-TE- Hi s e was incubated with ImM of ATP, 1 mM of Asp and 0.5 mM of Phe m a buffer (50 mM HEPES, 20 mM MgCl 2 , pH 8) for 6 hours at 37 °C. To finish the reactions, 100 μl of n- butanol were added to precipitate the enzyme which then was removed. The remaining clear solution was evaporated m vacuo for 3 hours and replenished to a volume of 20 μl with 10 vol% of methanol m water. For further analyses this volume was diluted lOx (again with 10 voi% metnanol m water) and portions thereof then were subjected to HPLC followed by photometric detection (as shown below) and to Electrospray Ionisation LC-MS analysis. Reference samples of Asp-Phe syntnesised chemically were used for comparison. For the HPLC with photometric detection a 50 μl portion of the lOx diluted sample was injected on a Chromsep Inertsil 5 ODS-3 column (250 x 3 mm; particle size 5 μ) . Eluents A (0.05 M aq. H 3 B0 3 buffer pH = 3.0) and B (aceto trile; Merck, HPLC-grade) were used at a gradient (t = 0 mm: 98% A, 2% B; t = 35 mm: 10% A, 90% B) and a flow of 1.2 ml/mm, at 40 °C . Detection was done photometrically (at 210 nm and 257 nm, with quantification at 210 nm) . In the resulting HPLC chromatogram for the experimental sample a peak was found at 6.59 minutes which was at exactly the same retention time as for the Asp-Phe reference compound. Moreover, the UV-spectrum recorded for the Asp-Phe peak from the sample was shown to be identical (as to extinction versus wavelength in the region from 200 to 380 nm) to the one recorded for the Asp-Phe reference compound. The amount of Asp-Phe in the sample was calculated to be 16.1 mg/1. For the Electrospray Ionisation LC-MS analysis a 4 μl portion of the lOx diluted experimental sample (see above) was injected onto the column. As a column a Nucleosil 120-3 C18 reversed-phase (Macherey & Nagel, 250 x 4 mm) was used. The eluents were eluents A (demi-water containing 0.05% of formic acid) and B (HPLC-grade methanol containing 0.05% of formic acid) . As a gradient eluents A and B were used as follows: t = 0: 10%B; t = 25 mm: 60% B; t = 30 mm: 100% B; t = 34 m : 100% B at a flow of 0.4 ml/mm at ambient temperature. Detection was done by total ion current (TIC) using electrospray ionisation m the positive ion mode as ionisation technique. The scan range was 120-300 amu with a dwell time of 1 msec. The retention time for Asp-Phe was 23.7 minutes (both for the sample and the reference compound) . Mass spectroscopy confirmed the same molecular weight (280 g/mol) for both the Asp-Phe from the sample and the reference compound, and also identical fragmentation patterns were observed. According to this technique the amount of Asp-Phe the sample was calculated to be about 20 mg/1.

Claims: CLAIMS 1. Method for the microbiological production of α-L- aspartyl-L-phenylalanme (Asp-Phe) from the substrates L-aspartic acid (L-Asp) and L- phenylalan e (L-Phe) characterised that the substrates are contacted, m the presence of an effective amount of adenosme-tπphosphate (ATP) , with a non-ribosomal dipeptide synthetase comprising two minimal modules connected by one condensation domain wherein the N-termmal module of these modules is recognising L-aspartic acid and the C-termmal module of these modules is recognising L-phenylalamne and is covalently bound at its N-termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' - phosphopantethemyl cofactor containing thiolation domain, and that the -L-aspartyl-L-phenylalanme (Asp-Phe) formed is recovered. 2. Method for the production of Asp-Phe according to claim 1, characterised m that the condensation domain in the dipeptide synthetase is connected to both minimal modules such way that it is also covalently bound to the module recognising L- aspartic acid. 3. Method for the production of Asp-Phe according to claim 1 or 2 , characterised that also a releasing factor is present for the Asp-Phe formed on the dipeptide synthetase. 4. Method for the production of Asp-Phe according to any of claims 1 to 3 , characterised in that the releasing factor is a protein which shows thioesterase-like functions and forms an integrated domain of tne dipeptide synthetase at the C-terminus thereof. 5. Method for the production of Asp-Phe according to any of claims 1 to 4 , characterised that prior to the production of Asp-Phe the dipeptide synthetase has undergone optimisation for its function by using one or more post-translational modifying activities. 6. Method for the production of Asp-Phe according to claim 5, characterised that as an post- translational modifying activity a 4 ' - phosphopantethemyl (4'-PP) transferase is used. 7. Method for the production of Asp-Phe according to any of claims 1 to 6, characterised that also a non- integrated protein with thioesterase Type-II- like activity is present together with the dipeptide synthetase. 8. Method for the production of Asp-Phe according to any of claims 1 to 7, characterised m that the dipeptide synthetase is present m living cell- material of a micro-organism, and that glucose, L- Asp and/or L-Phe are being fed to said fermentor, and that the Asp-Phe formed is recovered. 9. Method for the production of Asp-Phe according to claim 8 characterised m that the micro-organism is first grown m a fermentor to reach a predetermined cell density before the expression of the Asp-Phe dipeptide synthetase is switched on and feeding of the glucose, L-Asp and/or L-Phe for the synthesis of the Asp-Phe dipeptide is started. 10. Method for the production of Asp-Phe according to claim 9, characterised that the micro-organism is an L-phenylalanme producing micro-organism and that only glucose and L-Asp are being fed. 11. Method for the production of Asp-Phe according to claim 10, characterised in that the micro-organism is an Escherichia or Bacillus species. 12. Method for the production of Asp-Phe according to any of claims 8 to 11, characterised m that the micro-organism used is a strain with reduced protease activity for Asp-Phe or lacking such activity towards Asp-Phe. 13. Method for the production of Asp-Phe according to any of claims 8 to 12, characterised m that the micro-organism used also contains a suitable export system for Asp-Phe formed and/or one or more suitable up-take system (s) for glucose and/or L-Asp and/or L-Phe. 14. Method for the production of Asp-Phe according to any of claims 1 to 7, characterised in that the production of Asp-Phe is carried out in vi tro in an enzyme reactor, while ATP is supplied, and L- Asp and/or L-Phe are being fed, and the Asp-Phe formed is recovered. 15. Method for the production of Asp-Phe according to claim 14, characterised m that the supply of ATP is provided m part by an in situ ATP-regeneratmg system. 16. Method for the production of Asp-Phe according to claim 15, characterised m that the ATP- regeneratmg system is present a permeabilised micro-organism. 17. A DNA fragment or a combination of DNA fragments coding for a non-ribosomal Asp-Phe dipeptide synthetase, which synthetase comprises two minimal modules connected by one condensation domain wherein the N-termmal module of these modules is recognising L-aspartic acid and the C-termmal module of these modules is recognising L- phenyiaiamne and is covalently bound at its N- termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' -phosphopantethemyl cofactor containing thiolation domain. 18. A DNA fragment coding for an Asp-Phe dipeptide synthetase according to claim 17, characterised that the condensation domain m the encoded dipeptide synthetase is connected to both minimal modules such way that it is also covalently bound to the module recognising L-aspartic acid. 19. A DNA fragment according to claim 17 or 18, or a combination of DNA fragments according to claim 17, characterised m that the DNA fragment or combination of DNA fragments encoding the dipeptide synthetase also code for a releasing factor for the Asp-Phe formed on that dipeptide synthetase. 20. A DNA fragment or a combination of DNA fragments according to any of claims 17 to 19, characterised in that it/they also code for a protein which shows thioesterase-like functions and forms an integrated domain of the dipeptide synthetase at the C- terminus thereof. 21. A DNA fragment or a combination of DNA fragments according to any of claims 17 to 20, characterised m that it/they also express (es) a post- translational modifying activity for efficient non-ribosomal synthesis of Asp-Phe on the synthetase . 22. A DNA fragment or a combination of DNA fragments according to claim 21, characterised in that the post-translational modifying activity expressed is a 4 ' -phosphopantethemyl (4 ' -PP) transferase activity. 23. A DNA fragment or a combination of DNA fragments according to any of claims 17 to 22, characterised in that it/they also code for a non-integrated protein with thioesterase Type-II-like activity. 24. A micro-organism containing a DNA fragment or a combination of DNA fragments according to any of claims 17-23. 25. A micro-organism according to claim 24 wherein the micro-organism is capable of producing L-Asp and/or L-Phe. 26. A micro-organism according to claim 25 wherein the micro-organism is an Escherichia coli or Bacillus species . 27. Asp-Phe dipeptide synthetase characterised in that it comprises two minimal modules connected by one condensation domain wherein the N-termmal module of these modules is recognising L-aspartic acid and the C-termmal module of these modules is recognising L-phenylalamne and is covalently bound at its N-termmal end to the condensation domain, and wherein each of these minimal modules is composed of an adenylation domain and a 4 ' - phosphopantethemyl cofactor containing thiolation domain. 28. Asp-Phe dipeptide synthetase according to claim 27 characterised m that the condensation domain m the dipeptide synthetase is connected to both minimal modules m such way that it is also covalently bound to the module recognising L- aspartic acid. 29. Asp-Phe dipeptide synthetase according to claim 27 or 28, characterised that the dipeptide synthetase also comprises a releasing factor for the Asp-Phe formed o that dipeptide synthetase. 30. Asp-Phe dipeptide synthetase according to claim 29, characterised m that the releasing factor is a protein which shows thioesterase-like functions and forms an integrated domain of the dipeptide synthetase at its C- terminus.

Inventors: DOEKEL SASHA, MARAHIEL MOHAMED ABDALLA, QUAEDFLIEG PETER JAN LEONARD M, SONKE THEODORUS, DOEKEL, SASHA, MARAHIEL, MOHAMED, ABDALLA, QUAEDFLIEG, PETER, JAN, LEONARD, MARIO, SONKE, THEODORUS
Applicants: DOEKEL SASHA, HOLLAND SWEETENER CO, MARAHIEL MOHAMED ABDALLA, QUAEDFLIEG PETER JAN LEONARD M, SONKE THEODORUS, DOEKEL, SASHA, HOLLAND SWEETENER COMPANY V.O.F., MARAHIEL, MOHAMED, ABDALLA, QUAEDFLIEG, PETER, JAN, LEONARD, MARIO, SONKE, THEODORUS
Application year: 2000
Main classification: C12N15
IPC codes: C07K5, C12N9, C12N15