Abstract:
Genes are disclosed which are capable of directing the synthesis in a selected host microorganism of thaumatin I polypeptides having improved taste characteristics characterized by having the amino acid sequence of natural thaumatin I including a lysine amino acid residue substituted for asparagine in the 46th position from the amino terminal end, an aspartate amino acid residue substituted for asparagine in the 113th position from the amino terminal end and an aspartate amino acid residue substituted for lysine at either the 97th or 137th residue from the amino terminal end.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 07/268,702 filed Nov. 8, 1988, now abandoned the disclosure of which is herein incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates generally to the manipulation of genetic materials and more particularly to the manufacture of specific DNA sequences useful in recombinant procedures to secure the production of improved variants of a polypeptide identified as natural thaumatin I. 
     Thaumatin is an extremely sweet-tasting protein produced in the arils of the fruit of the African shrub Thaumatococcus daniellii Benth. The fruit traditionally has been used in West Africa as a sweetener of palm wine, corn bread, and sour fruit. Thaumatin, which is about 5,000 times sweeter than sucrose on a weight basis, is produced in at least five forms: thaumatins I, II, a, b and c. These proteins, named in the order of elution from an ion exchange column [Higginbotham, et al., in Sensory Properties of Foods (Birch, et al., eds.), London: Applied Sciences, pp. 129-149 (1977)], have molecular weights of approximately 22 kilodaltons. Thaumatins I and II are nearly identical proteins, each consisting of a single unmodified polypeptide chain, 207 amino acid residues in length. 
     Thaumatin is a non-toxic, low-calorie and noncariogenic protein which elicits profound sweet taste responses. These properties suggest a stable interaction between the proteins and human taste receptors. Therefore, thaumatin has potential for use as a sugar substitute, food additive, a sweetness receptor probe and a tool for further elucidation of the taste response. 
     A plentiful supply of pure thaumatin is required to utilize the protein as a possible food additive and research tool. Because the thaumatin plant requires a tropical climate and insect pollination for successful fruit propagation, there are considerable difficulties involved in greenhouse cultivation of the fruit. 
     Iyengar disclosed an amino acid sequence for thaumatin I which is shown in Table 1 below [Iyengar, et al., Eur. J. Biochem., 96, 193-204 (1979)]. 
     
                                           TABLE 1__________________________________________________________________________ ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ##STR7## ##STR8## ##STR9## ##STR10## ##STR11## ##STR12## ##STR13## ##STR14## ##STR15##__________________________________________________________________________ 
    
     The amino acid sequence for thaumatin II has been deduced from its nucleotide sequence [Edens, et al., Gene, 18, 1-12 (1982)] and a gene for thaumatin II has been cloned from messenger RNA-derived cDNA. The five amino acids in the thaumatin II sequence which differ from the thaumatin I sequence above are the following: lysine instead of asparagine at residue 46; arginine instead of serine at residue 63; arginine instead of lysine at residue 67; glutamine instead of arginine at residue 76; and aspartic acid instead of asparagine at residue 113. Sequence analysis also indicated that thaumatin II is initally translated as a precursor form, preprothaumatin, with both a 22 residue amino-terminal extension and an acidic, six-amino acid carboxy terminal tail. The amino terminal peptide was postulated as a secretion signal based on its hydrophobic character and a compartmentalization role was hypothesized for the carboxy terminal extension. 
     The Edens, et al. reference cited above notes that a polypeptide having the native sequence of preprothaumatin II has been microbially produced. More specifically, the reference and Verrips, et al., U.S. Pat. No. 4,771,000 disclose cDNA sequences coding for native mature thaumatin II and preprothaumatin II and also disclose cloning vehicles comprising the DNA sequences for use in transformation in microorganisms. 
     In co-owned and copending U.S. patent application Ser. No. 540,634 filed Oct. 11, 1983, the disclosure of which is herein incorporated by reference, techniques for the synthesis of manufactured genes coding for the amino acid sequence of thaumatin I as identified by Iyengar, et al. were disclosed, as were DNA microorganism transformation vectors, fusion genes, transformed microorganisms, and processes for expressing the manufactured gene and for securing the polypeptide product produced thereby. Specific manufactured genes of the application incorporated a number of codons &#34;preferred&#34; for expression in yeast host cells. 
     In co-owned and copending U.S. patent application Ser. No. 189,250, a continuation of Ser. No. 797,474 filed Nov. 13, 1985, the disclosure of which is hereby incorporated by reference, polypeptides and genes for their synthesis were disclosed having the amino acid sequence of [Asp 113  ] thaumatin I and [Lys 46 , Asp 113  ] thaumatin I, i.e., containing the continuous sequence of amino acid residues of natural thaumatin I as reported by Iyengar, et al. except for an aspartate amino acid residue substituted for asparagine in the 113th position from the amino terminal end of the polypeptide and optionally a lysine amino acid residue substituted for asparagine in the 46th position. Those polypeptides could be folded to a sweet conformation using an in vitro procedure. Recombinant produced thaumatin I having the published Iyengar, et al. amino acid sequence was not sweet and could not be folded to a sweet conformation. 
     While the recombinant produced [Asp 113  ] and [Lys 46 , Asp 113  ] thaumatin I polypeptides constitute improvements over the art, they share with plant derived thaumatin taste characteristics which may limit their use in some food products. Most significant of these characteristics is a lingering aftertaste unlike that exhibited by sugar (sucrose). 
     Of interest to the present invention is the disclosure of van der Wel, et al., Chemical Senses and Flavor, 2, 211-218 (1976). This reference discloses experiments in which lysine residues in thaumatin I were chemically modified by acetylation with acetic anhydride and by reductive methylation. At least four acetylated thaumatins were obtained with either one, two, three or four acetylated amino groups. In addition, a methylated thaumatin was produced having six dimethyl lysine residues and one monomethyl lysine residue. While the methylated thaumatin with seven modified lysine residues had a sweetness intensity practically equal to that of the original thaumatin, the sweetness intensity of the acetylated thaumatins decreased with the increasing number of acetylated amino groups. The sweet taste sensation disappeared completely when four acetylated lysine residues were introduced into the molecule. The reference suggested the possibility that a correlation between net charge and the sweetness intensity of the molecule may exist. 
     Of interest to another aspect of the present invention are references relating to polypeptide secretion signal sequences. Von Heijne, European J. Biochem., 133, 17-21 (1983), discloses a variety of eukaryotic signal sequences and notes that signal sequences typically contain two different, independent signals. The first signal is said to be in the form of a hydrophobic core while the second one is said to confer processing specificity. Oliver, Ann. Rev. Microbiol., 39, 615-48 (1985), relates to protein secretion in Escherichia coli and discloses a variety of bacterial secretion signals which are characterized as including three conserved features including (a) a positively charged amino terminus, (b) a hydrophobic core and (c) a highly conserved sequence adjacent to the processing site. 
     Kendall, et al., Nature, 321, 706-708 (1986), discloses the use of site-directed mutagenesis to produce a mutant signal sequence containing nine consecutive leucine residues in the hydrophobic core segment Sjostrom, et al., The EMBO Journal, 6, 823-831 (1987), relates to analysis of E. coli secretion signal sequences and discloses the presence of lysine residues in the amino terminal end of the sequence for a high number of E. coli signal sequences. Sjostrom, et al. note, however, that the signal peptides of eukaryotes rarely have sequence homologies. 
     SUMMARY OF THE INVENTION 
     The present invention provides purified and isolated DNA sequences capable of directing synthesis in a selected host microorganism of [Lys 46 , Asp 97 , Asp 113  ] thaumatin I and [Lys 46 , Asp 113 , Asp 137  ] thaumatin I containing the continuous sequence of amino acid residues of natural thaumatin I as reported by Iyengar, et al. except for a lysine amino acid residue substituted for asparagine in the 46th position from the amino terminal end of the polypeptide, an aspartic acid residue substituted for asparagine in the 113th position and either aspartic acid for lysine in the 97th position or aspartic acid for lysine in the 137th position. In preferred forms of the genes, the base sequences include one or more codons specifying the same amino acid on the basis of preferential expression characteristics of the codon in a projected host microorganism, e.g., E. coli, Saccharomyces cerevisiae. Other preferred forms of manufactured genes include those wherein: (1) a codon specifies an additional amino acid in the polypeptide synthesized (e.g., an initial methionine residue) which facilitates direct expression in  E. coli microorganisms and/or yeast microorganisms; or (2) at least one termination codon at the end of the manufactured gene to insure proper termination of the polypeptide. 
     In practice of the invention to generate polypeptide products, DNA sequences, including manufactured genes, are inserted into a viral or circular plasmid DNA vector to form a hybrid vector and the hybrid vectors are employed to transform host microorganisms such as bacteria (e.g., E. coli) or yeast cells (e.g., S. cerevisiae). Vectors may also be supplied with appropriate promoter/regulator DNA sequences, allowing for controlled expression in the host microorganism. The transformed microorganisms are thereafter grown under appropriate nutrient conditions and express the polypeptide products of the invention. 
     The invention also provides novel DNA sequences encoding improved yeast secretion signal sequences which decrease the lag time required for transformants to appear for recombinant produced polypeptides such as thaumatin. In addition, the invention further provides preferred mutagenized host cells for expression and secretion of the thaumatin proteins of the invention. 
     The novel [Lys 46 , Asp 97 , Asp 113  ] and [Lys 46 , Asp 113 , Asp 137  ] thaumatin I polypeptides provide novel taste characteristics, particularly with respect to reduced longevity of sweet aftertaste. This invention teaches the importance of the 97th and 137th amino acid residues to the biological properties such as taste characteristics of the thaumatin I molecule. It is expected that additional thaumatin I polypeptides having improved taste characteristics can be produced by substitution of amino acids other than aspartic acid for lysine at either position 97 or 137. Other aspects and advantages of the present invention will be apparent upon consideration of the following detailed description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a depiction of the relative taste qualities of [Lys 46 , Asp 97 , Asp 113  ] and [Lys 46 , Asp 113 , Asp 137  ] thaumatin I polypeptides and plant derived thaumatin. 
     FIG. 2 is a depiction of the relative taste qualities of [Lys 46 , Asp 97 , Asp 113  ] and [Lys 46 , Asp 113 , Asp 137  ] thaumatin I polypeptides and plant derived thaumatin. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLE 1 
     In this example, plasmids pING152T encoding [Lys 46 , Asp 113 , Asp 137  ] thaumatin I and pING323T encoding [Lys 46 , Asp 97 , Asp 113  ] thaumatin I were constructed from plasmids pKS6 and M13mp10-thaumatin, the constructions of which are described in co-owned and copending U.S. patent application Ser. No. 189,250 filed May 2, 1988, the disclosure of which is hereby incorporated by reference, and pING58T, the construction of which is described in published international application No. US/PCT86/02443 corresponding to, copending U.S. patent application Ser. No. 797,477 filed Nov. 13, 1985, the disclosure of which is hereby incorporated by reference. 
     Plasmid pKS6, which contains the 218 bp PGK promoter and encodes for a [Lys 46 , Asp 113  ] thaumatin I polypeptide, was cut with BamHI and XhoI and the 850 base pair PGK promoter-thaumatin fragment was ligated to BamHI and XhoI cut M13MP10-thaumatin to form plasmid M13-KS6. The single-stranded phage DNA was prepared from M13-KS6 and used as template for site-directed mutagenesis. 
     Two oligonucleotides, 5&#39;-GAGCCTTAAGGTCAGCTGGACAT-3&#39; for changing Lys 137  to Asp and 5&#39;-GATGTAGTCGTCACCGTATTG-3&#39; for changing Lys 97  to Asp were annealed to M13-KS6, separately, and site-directed mutagenesis was performed as described in Miyada, et al., Gene, (1982), 17:167-177. The same oligonucleotides used for mutagenesis were used as probes to screen for positive plaques. Positive plaques were sequenced by the chain termination reaction (Sanger, et al., Proc. Natl. Acad. Sci. USA, (1977), 74:5463-5467) and the desired changes were confirmed. One positive plaque from each mutagenesis was picked. Double-stranded replicated-form DNA was prepared, cut with BamHI and XhoI, ligated to BamHI and XhoI cut pING58T to form pING152T [Lys 137  to Asp 137  ] and pING323T [Lys 97  to Asp 97  ]. These two constructs upon transformation into yeast strain BB29-lc produced intracellular [Lys 46 , Asp 113 , Asp 137  ] thaumatin I and [Lys 46 , Asp 97 , Asp 113  ], thaumatin I, respectively. The thaumatin polypeptides were purified from yeast cell extracts as described in co-owned and copending U.S. Pat. No. 4,766,205, the disclosure of which is hereby incorporated by reference, and each refolded to a sweet form. 
     EXAMPLE 2 
     In this example, plasmids pING152CVS and pING323CVS were constructed and separately transformed into yeast strains. A yeast secretion vector, plasmid pING804CVS, which contains the 220 bp PGK promoter, the SUC2 yeast invertase signal sequence, the structural sequence for [Lys 46 , Asp 113  ] thaumatin I from which a 138 base pair deletion has been made and the PGK terminator on a medium copy yeast vector CV20 (YEp20, ATCC 37059, Broach, Methods in Enzymology, 101:307-325) was used as the base vector for constructing the YT152 and YT323 secretion vectors. The reason for choosing pING804CVS as a base vector is because it generates a smaller BglII - XhoI fragment and can be easily distinguished from the full size BglII - XhoI fragment of the YT152 and YT323 genes. Plasmids pING152T and pING323T were cut with BglII and XhoI and the 485 bp partial thaumatin genes ligated to BglII and XhoI cut pING804CVS to generate pING152CVS and pING 323CVS, respectively. 
     Plasmids pING152CVS and pING323CVS were transformed into yeast strain AH7 (MATα leu2-3), (obtained from the laboratory of Gerald Fink, Whitehead Institute of Cancer Research) separately. Leu +  transformants were grown in SD-leu for 4 days and samples taken for radioimmunoassay. The secretion level of YT152 or YT323 was determined to be approximately 0.4 μg/ml. 
     EXAMPLE 3 
     As an alternative means of producing vectors encoding the thaumatin polypeptides of the present invention, it is well within the skill in the art to synthesize using commercially available devices entire nucleotide sequences encoding the amino acid sequences of the thaumatin polypeptides of the invention along with suitable promoter, secretion signal and terminator sequences. Cassettes comprising such sequences may be provided at their 5&#39; and 3&#39; ends with nucleotides duplicative of restriction sites in order that they may be readily inserted into suitable plasmids. 
     For example, provided below in Tables 2 and 3 are nucleotide sequences of DNA cassettes encoding the pING152CVS sequence comprising PGK promoter - SUC2 signal sequence - [Lys 46 , Asp 113 , Asp 137  ] thaumatin I -PGK terminator (Table 2) and the pING323CVS sequence comprising PGK promoter - SUC2 signal sequence - [Lys 46 , Asp 97 , Asp 113  ] thaumatin I - PGK terminator (Table 3). These sequences also include &#34;sticky ends&#34; to a BamHI site at the 5&#39; end and a SalI site at the 3&#39; end and when inserted into publicly available plasmid CV20 (YEp20, ATCC 37059) opened with the BamHI and SalI restriction enzymes produce alternatively pING152CVS and pING323CVS. 
     
                                           TABLE 2__________________________________________________________________________pING152CVS Sequence__________________________________________________________________________ ##STR16## ##STR17## ##STR18## ##STR19## ##STR20## ##STR21## ##STR22## ##STR23## ##STR24## ##STR25## ##STR26## ##STR27## ##STR28## ##STR29## ##STR30## ##STR31## ##STR32## ##STR33## ##STR34## ##STR35## ##STR36## ##STR37## ##STR38## ##STR39## ##STR40## ##STR41## ##STR42##__________________________________________________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________pING323CVS Sequence__________________________________________________________________________ ##STR43## ##STR44## ##STR45## ##STR46## ##STR47## ##STR48## ##STR49## ##STR50## ##STR51## ##STR52## ##STR53## ##STR54## ##STR55## ##STR56## ##STR57## ##STR58## ##STR59## ##STR60## ##STR61## ##STR62## ##STR63## ##STR64## ##STR65## ##STR66## ##STR67## ##STR68## ##STR69##__________________________________________________________________________ 
    
     EXAMPLE 4 
     Purification from Growth Media 
     In this example methods are described for purification of thaumatin produced by transformed cells and secreted into growth medium. According to methods for purification of thaumatin from cultures grown in shaker flasks (0.2 to 20 liters) or in small scale fermentors (10 to 75 liters), culture medium is clarified by continuous flow centrifugation (Westfalia SA-1) to pellet yeast cells. 
     The cell-free growth medium is then concentrated ten to twenty-fold using an Amicon DC-10L concentrator fitted with a 10,000 dalton molecular weight cutoff spiral cartridge and washed with 15 volumes of water. The concentrate is then centrifuged at 17,000×g for 20 minutes in order to clarify (with hollow fiber cartridges used for large volumes.) The supernatant is then filtered through a 0.45 μm pore size membrane in order to remove particulates and remaining cell debris. Sodium phosphate buffer is then added to a concentration of up to 10 mM in order to adjust the pH to between 7 and 8. 
     The clarified growth medium concentrate is then loaded onto a Zeta Chrom-60 SP disc (Western Analytical, sulfopropyl cation exchanger). The disk is washed with buffer, 10 mM sodium or potassium phosphate buffer pH 7-8, and the flow-through discarded after testing for the presence of thaumatin by radioimmunoassay. The thaumatin concentrated on the SP-resin is then eluted from the disc with 200 ml of a 250 mM NaCl solution in the buffer used previously. The dilute thaumatin solution is then concentrated by pressure ultrafiltration on an Amicon stirred cell fitted with a YM-5 membrane and washed twice with water to yield a substantially purified product (i.e., greater than 95%). 
     When thaumatin is purified from growth media from a larger scale fermentation (i.e., 400 to 600 liters) a modified purification procedure is followed. The fermentation medium is first subjected to an inactivation procedure comprising treatment with phosphoric acid to reach pH 4.0, addition of 0.4% sodium benzoate (w/v) and incubation at 30° C. for 30 minutes with no aeration. The cells were pelleted by centrifugation then killed with 5% bleach. 
     The cell-free growth medium is then concentrated ten to twenty fold using an Amicon DC-30P concentrator fitted with six 10,000 molecular weight cutoff spiral cartridges and washed with five volumes of water. The medium is then clarified within an Amicon hollow filter cartridge with a 0.1 μm cut off and washed with 15 liters of 250 mM NaCl in 10 mM sodium phosphate, pH 7-8. At this time the medium is concentrated using an Amicon DC-10L concentrator, centrifuged at 17,000×g, filtered through a 0.45 μm millipore membrane and buffered to pH 7 to 8 in the same manner as described for the smaller scale fermentations. 
     The clarified growth medium concentrate is then loaded onto a Zeta Chrom (Western Analytical) 100 cc SP capsule at a maximum flow rate of 20 ml per minute and washed with 10 mM sodium or potassium phosphate buffer pH 7-8. The concentrated protein is then eluted using one liter of 250 mM NaCl in the buffer. The eluate is monitored for protein elution with the Bio Rad Protein Assay. The protein positive fractions are then pooled and desalted/concentrated by ultrafiltration using an Amicon stirred cell fitted with a YM-5 membrane to yield a substantially purified product. 
     EXAMPLE 5 
     In this example, various improved yeast secretion signal sequences were constructed. Plasmid pKS48 was constructed for introducing sequence changes (primarily to encode positively charged amino acids) into the 5&#39;-end of the SUC2 wild type yeast invertase signal sequence. Plasmid pSH4 which contains the 218 bp PGK promoter with SphI - SstI - XhoI linkers at the 3&#39;-end was digested with XhoI, filled-in with T4 DNA polymerase and then digested with BamHI. The BamHI-blunt-end 218 bp PGK promoter fragment was ligated to HindIII digested, T4 polymerase blunt-ended, BamHI cut pKK108 to generate pKS48. Plasmid pKS48 contains the 218 bp PGK promoter, a partial invertase signal sequence and the partial thaumatin gene. 
     Digesting pKS48 with SphI and HindIII allows insertion of synthetic oligonucleotides to change the 5&#39;-end of the invertase signal sequence. Five oligonucleotides were synthesized. The amino acid sequence changes they code for are shown in Table 4 below. 
     
                                           TABLE 4__________________________________________________________________________ ##STR70## ##STR71## ##STR72## ##STR73## ##STR74##__________________________________________________________________________ 
    
     Each synthetic oligonucleotide was ligated to plasmid pKS48 that had been cut with SphI and HindIII. Upon transformation into E. coli strain MC1061, the single-stranded gaps were repaired in vivo (Huang, et al., Biochemistry, (1987), 26:8242-8246) to form plasmids pKS48-1 [Leu(1)→Arg], pKS48-2 [Leu(2)→Arg], pKS48-3 [Gln(3)→Lys], pKS55 [Leu(2)→LysLysLys), and pKS56 [Leu(2)→LysArgLysArg]. The BamHI-EcoRI fragments which contain the 218 bp PGK promoter-mutated signal sequence-partial thaumatin gene sequence were purified from all five plasmids individually. Also, an EcoRI-XhoI fragment which contains the 3&#39;-half of a gene designated YT406 encoding [Lys 46 , Asp 113  ] thaumatin I was purified from pING95CVS or pING438CVS (although the YT406 gene could have been obtained from other plasmids such as pKS6). The BamHI-EcoRI fragments from pKS48-1, pKS48-2 and pKS48-3 were ligated to the EcoRI-XhoI fragment from pING95CVS and BamHI-XhoI digested pING58 (a plasmid containing a PGK terminator sequence, the construction of which is described in co-owned and copending U.S. patent application Ser. No. 189,250 filed May 2, 1988) to form pING447, pING448 and pING449, respectively. The BamHI-EcoRI fragments from pKS55 and pKS56 were ligated to the EcoRI-XhoI fragment from pING438CVS and BamHI-XhoI digested pING58 to form plasmids pING454 and pING456, respectively. 
     To increase hydrophobicity in the center portion of the SUC2 signal sequence, a synthetic oligonucleotide (5&#39;GCTTTCTTGATCTTGTTGATTTTGTTCCCAGGT AAGATCTCTGCCTGCA 3&#39;) was used to replace the small HindIII-PstI fragment of pKS46 by the in vivo gap-filling technique to generate pKS49. The thaumatin gene from pING174 was joined as an in-phase fusion to the new signal sequence of pKS49 by a two step cloning to form pKS54. The 218 bp PGK promoter-signal sequence-thaumatin gene was cloned into pING58 as a BamHI-XhoI fragment to generate pING451. In plasmid pING451, the following substitutions in the SUC2 secretion signal sequence were made: [Phe 7  →Ile, Ala 10  →Ile, Gly 11  →Leu, Ala 13  →Pro and Ala 14  →Gly]. 
     Combining the changes in pKS55 and pKS56 with those in pKS54 was done as follows. The small BamHI-HindIII fragments from pKS55 and pKS56 were ligated with the small HindIII-KpnI fragment from pKS54 into BamHI-KpnI cut pKS6 to form pKS57 and pKS58,  respectively. The BamHI-XhoI fragments (about 920 bp, containing the promoter - signal - thaumatin fusions) from pKS57 and pKS58 were then cloned into BamHI and XhoI cut pING58 to form pING460 and pING462, respectively. These plasmids encoded secretion signal sequences with both positively charged amino acid residues at the amino end of the sequence and increased hydrophobicity in the center of the molecule. 
     Yeast strain BB33-lc [MATa, leu 2-3, 2-112, ura3-52] was transformed with plasmids pING156, pING447, pING448, pING449, pING451, pING454, pING456, pING460 and pING462. The Leu +   transformants were selected on SD-leu plates and single colonies picked into SD-leu broth and grown for 4 days. Samples were then taken for radioimmunoassay. The maximum yield of [Lys 46 , Asp 113  ] thaumatin I production compared to the transformation lag time was recorded in Table 5 below. 
     
         TABLE 5  Effect of Modified Signal Sequences on Thaumatin Secretion Max.  Yield Lag Time (μg/ml) (days)   Wild Type Sequence, pING156  ##STR75##  1.0 20-30  pING447, Leu to Arg at 1 ##STR76##  1.8 10-14  pING448, Leu to Arg at 2 ##STR77##  0.3 10-14  pING449, Gln to Lys at 3 ##STR78##  0.4 10-14  pING451, 5 amino acids changes ##STR79##  0.4 10-14  pING454 ##STR80##  1.6 10-14  pING456 ##STR81##  1.0 4-5  pING460 ##STR82##  1.1 2-4  pING462 ##STR83##  1.3 2-4 
    
     EXAMPLE 6 
     In this example, PS16, a preferred yeast strain that secretes higher quantities of preferred thaumatin was isolated. Strain BB33-lc (MATa leu2-3,2-112 ura3-52) was transformed with pING156 (a high copy number expression plasmid encoding [Lys 46 , Asp 113  ] thaumatin I and comprising a wild type secretion signal sequence) by selecting for Leu +   transformants. A small fraction of the transformants secreted 0.3-0.6 μg/ml of thaumatin. One of these higher secreting transformants was later found to be no longer haploid and was designated PS11. PS11 containing pING156 was mutagenized with N-methyl-N&#39;-nitronitrosoguanidine, and colonies were screened for increased secretion. 
     Screening involved growing colonies on SD-leu plates, lifting the colonies from the master plates onto nitrocellulose filters, and placing the filters colony side up onto fresh SD-leu plates. After 6 to 24 hours of growth, the colonies were washed from the filters, and the filters were checked for thaumatin by protein immunoblotting (Western procedure). Higher secreting colonies were picked from the master plate and rechecked by Western and RIA procedures. This approach was repeated by serially isolating higher secreting colonies to give the genealogy PS11[pING156]→PS6[pING156]→PS10[pING156]→PS14[pING156]. PS14 was cured of the plasmid by growing on non-selective medium (YPD) and retransformed with pING460. Mutagenesis of PS14[pING460] yielded PS16[pING460]. Yeast strain PS16 is on deposit under contract with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland 20852-1776 as A.T.C.C. Deposit No. 20909. The thaumatin secretion of each mutant in the series is shown in Table 6 below. 
     
                       TABLE 6______________________________________[Lys.sup.46, Asp.sup.113 ] THAUMATIN ISECRETION BY MUTANTS CONTAININGVECTORS WITH DIFFERENT SIGNAL SEQUENCES(μg/ml) assayed by RIApING         pING     pING     pING   pING462          460      456      156    94______________________________________BB33-1C 2.9      &gt;3.8     0.3    --     0.3PS11    5.0      5.4      1.4    0.6    --PS4     --       --       --     1.6    1.2PS5     --       --       --     2.0    0.9PS6     &gt;7.7     3.9      --     4.0    0.4PS10    6.8      7.7      --     5.0    --PS16    --       10.0     --     --     --______________________________________ 
    
     Construction of pING816A 
     Genomic DNA from Kluyveromyces lactis strain KG51-5A (ATCC #48792) was partially digested with the restriction enzyme Sau3A. Fragments 10-12 Kb in size were fractionated and collected on sucrose gradient. This DNA was ligated into pING1001RKnB that had been cut with BamHI (BamHI ends are compatible with Sau3A ends) to form a yeast genomic plasmid library. pING1001RKnB is a S. cerevisiae - E. coli shuttle vector containing the Tn903 aminoglycoside phosphotransferase gene linked to the S. cerevisiae 3-phosphoglycerate kinase (PGK) promoter. This allows transformation of the library into S. cerevisiae by selecting for resistance to the aminoglycoside G418. Plasmid pING1001RKnB is not critical to this procedure and may be replaced by other plasmids that have one or more markers complementing the host deficiency. 
     When the library was transformed into the yeast strain BB33-lc (MATa leu2-3, 2-112, ura3-52), a Ura +   transformant was isolated. Plasmid DNA was isolated from this transformant, transformed into E. coli, and harvested in a plasmid preparation. Although the BamHI site of pING1001RKnB is often destroyed when ligated with Sau3A ends (as it was in this case), it is flanked by two EcoRI sites (the only EcoRI sites in pING1001RKnB), which allows the DNA conferring Ura +   to BB33-lc to be removed as a 2.8 kb EcoRI fragment. 
     This 2.8 kb EcoRI fragment was blunt-ended by T4 DNA polymerase and ligated to the AatII cut, T4 DNA polymerase blunt-ended pING460 ([Lys 46 , Asp 113  ] thaumatin I) described previously. Plasmids with either orientation of the K. lactis URA3 fragment were isolated as determined by the placement of an XhoI site at one end of the fragment with respect to the PstI site of the Amp R  gene in the bacterial sequences of pING460. pING816A has the XhoI site about 1 kb from the PstI site, whereas pING816B has the XhoI site about 3 kb from the PstI site. 
     Plasmid pING816A transformants of yeast strain PS16 (a mutagenized derivative of BB33-lc), selected for Ura + , showed up in 3-4 days. They secreted about 10 μg/ml of thaumatin under Ura +   selection in liquid media. This level of thaumatin secretion was reproducible and appeared to be fairly stable. Selection for a Leu +   or a Leu +  Ura +   phenotype with pING816A yielded lower levels of thaumatin secretion (2-3 μg/ml). About 37% of the cells were still Ura +  after about 20 generations in non-selective YPD medium. 
     EXAMPLE 7 
     In this example, thaumatin proteins produced from cells transformed with plasmids pING152T and pING323T {yeast thaumatin 152 (YT152) [Lys 46 , Asp 113 , Asp 137  ] thaumatin I and yeast thaumatin 323 (YT323) [Lys 46 , Asp 97 , Asp 113  ] thaumatin I, respectively} were tested by a taste panel and compared with plant derived thaumatin. Plant thaumatin is intensely sweet, but has a delayed onset followed by a rapid rise in sweetness intensity, a long lasting sweetness and a lingering aftertaste which has been described as &#34;licorice-like.&#34; Higginbotham, et al., Flavor Potentiating Properties of Talin Sweetener (Thaumatin). The Quality of Foods and Beverages, Academic Press, New York, 91-111 (1981). 
     Fifteen potential subjects were screened for their ability to discriminate the basic tastes of sweet, sour, salty and bitter (2% sucrose, 0.07% citric acid, 0.2% sodium chloride,; 0.07% caffeine). Thirteen subjects (10 female and 3 male) successfully completed the screening program. 
     Following screening, the subjects participated in five, thirty minute sessions of training in descriptive analysis. The training included both round table discussion and testing in the partitioned sensory booths on samples selected to include sensory attributes and intensities representative of the sweeteners to be evaluated. 
     Training was completed once the subjects developed a descriptive sensory language with definitions, a scoresheet to measure the intensity of each characteristic, a standard procedure for examining and evaluating the products and demonstrated an ability to evaluate products using a scoresheet relating to eighteen flavor and aftertaste characteristics. (See Table 7 below.) 
     
                       TABLE 7______________________________________Flavor and AftertasteCharacteristics of Thaumatin Proteins______________________________________WHILE IN MOUTHInitial Blandness:           The bland non-sweet taste right           after the sample is put in the           mouth.Sweet:          The intensity of cane sugar           type sweetness.Bitter:         The taste stimulated by           caffeine.Salt:           The taste stimulated by table           salt (sodium chloride).Sour:           The taste stimulated by citric           acid.Caramelized Lactose:           The odor sensation on the back           of the throat or nose similar           to the odor of caramelized           lactose or Eagle brand           sweetened condensed milk.Menthol Cool:   The cooling sensation in the           mouth or nose produced by           menthol or mint.Licorice:       The taste stimulated by black           licorice.Medicinal:      The odor on the back of the           throat or nose similar to cough           syrup.Mouth Coating:  Degree of coating perceived in           the mouth after expectoration.AFTERTASTE:     The sensations following           expectoration.Sweet:          The residual sweetness after           expectoration.Licorice:       The residual licorice taste           after expectoration.Bitter:         The residual bitter taste after           expectoration.Sour:           The residual sour taste after           expectoration.Medicinal       The residual medicinal odor           after expectoration.Caramelized Lactose:           The residual caramelized           lactose odor after           expectoration.Metallic:       The aftertaste similar to the           taste of syrup of canned           pineapple.Mouth Watery:   The degree of salivation after           expectoration.Lingering:      The duration of sensation           remain in the mouth after           expectoration.Intensified with Water:           The residual sensations           reactivated with water.______________________________________ 
    
     Test samples were prepared in 1 liter isosweet (10% sucrose equivalent) masterbatches and held at 38° F. for from one to three days for testing. 
     
                       TABLE 8______________________________________Product      Concentration                     RIA Activity______________________________________Plant thaumatin        0.01 g/250 ml                     49 μg/mlYT152        Calc. @ 95 μg/ml                     90 μg/mlYT323        Calc. @ 96 μg/ml                     145 μg/ml______________________________________ 
    
     All testing was conducted in sensory booths which are designed to provide a controlled testing environment. Samples (10 mls) were served in 1 oz. odorless plastic cups. Each subject evaluated one sample at a time, and was provided with a 3 minute rest interval between samples. The subjects were asked to expectorate the samples, and to rinse with purified water and unsalted crackers between samples. All testing was done between 10:00 and 11:30 a.m. Samples were served to each subject coded only with three-digit random numbers. The numbers were different for each subject and each sample on each test. 
     Samples were served in a balanced block design, balanced for serving order, subject and day over the entire study. Each sample was evaluated by each of the thirteen subjects, once a day for three days, yielding approximately 39 observations per product on each of the eighteen attributes. The data were analyzed with a one-way analysis of variance to evaluate individual subject performance on each attribute, a two-way analysis of variance to evaluate overall panel performance on each attribute, and a Duncan Multiple Range Test to identify statistically significant differences between sample means. 
     Testing was also conducted with respect to the intensity of sweetness over time of the thaumatin proteins. Potential subjects were recruited and screened for their ability to detect sweetness in five suprathreshold concentrations of sucrose and plant thaumatin. Those who passed this initial screening were then tested for their ability to rank various concentrations of sucrose solutions in order from lowest to highest (5, 7.5, 10, 12.5 and 15% in water) according to the procedure of Swartz, et al., Food Technology, Vol. 31, 51-67 (1977). The thirteen to fifteen most accurate and reliable judges were selected to participate in the time-intensity studies. 
     Three practice sessions were conducted to give subjects basic instructions on the techniques of tasting and the method of operating the strip-chart recorder as described by Larson-Powers and Pangborn, J. Food Science, 43(1), 41-46 (1978). The subjects were then ready to being the time-intensity studies. 
     The sweetener concentrations were approximately equisweet with 10% (w/v) sucrose solution. These concentrations were derived at through the completion of isosweetness studies which were conducted to insure reliable time-intensity data. Subjects were given randomly numbered (3-digit) 10 ml samples of each sweetener, in a sequential monadic balanced order. The subjects were instructed to take the entire sample into their mouth, start the moving chart recorder with a foot pedal, and &#34;mark along the line that best reflects the relative sweetness intensity of the sample.&#34; A line was marked on the paper at which point the subjects were instructed to expectorate the sample (ten seconds) and continue scoring sweetness intensity until all sweetness was gone. The subjects cleansed their mouth using water, unsalted crackers, and a few minutes time in between sample evaluations. This procedure was repeated for each sweetener being tested. Each study was replicated three times to help ensure consistency, increase statistical validity, and provide a high quality, sufficiently large data base on which to arrive at reliable conclusions. 
     The following four sensory characteristics from each individual recording were measured: 
     1. Intensity at Expectoration: The numerical intensity value, on a 100 unit scale at 10 seconds. 
     2. Maximum Intensity: The highest sweetness intensity value scored, on a 100 unit scale. 
     3. Time to Maximum: The number of seconds elapsed from time zero until the time it reaches maximum sweetness. 
     4. Total Duration: The number of seconds from time zero until all sweetness is gone and recording is stopped. 
     These data were analyzed with a one-way analysis of variance to evaluate individual panelist performance, a two-way analysis of variance to evaluate overall panel performance on each measurement, and a Duncan Multiple Range test to identify statistically significant differences between the sweeteners. All statistical analysis were performed at the 95% level of confidence. 
     Mean values were also calculated for each 2.5 second interval along the charts generated by each judge for each sweetener. 
     The testing obtained the results that YT152 and YT323 were not significantly different from plant produced thaumatin with respect to salty, sour, menthol cool, medicinal, medicinal aftertaste and menthol aftertaste. There were significant differences, however, on eleven of the eighteen attributes measured. Both YT152 at 90 μg/ml and YT323 at 145 μg/ml were equally sweet with plant thaumatin at 49 μg/ml. YT323 was most similar to thaumatin, but left a less sour and less bitter aftertaste. YT152 was quite different from thaumatin. YT152 had less licorice flavor and aftertaste, a less sour and less sweet aftertaste, was less mouth watering, had less of a mouth coating and had less overall lingering sweetness (FIGS. 1 and 2, Tables 7-9). 
     In separate studies, YT152 at 104 μg/ml and YT323 at 70 μg/ml were equally sweet with 49 μg/ml plant thaumatin, took less time to reach maximum sweetness intensity and had less lingering sweetness than plant thaumatin (Table 12). 
     These data clearly indicate that the latent sweetness onset and the lingering aftertaste of plant thaumatin has been significantly reduced for both YT152 and YT323. YT152 also has a significantly reduced &#34;licorice-like&#34; flavor and aftertaste which may broaden its potential applications. 
     
                       TABLE 9______________________________________Lingering              Sweet______________________________________Thaumatin 29.77.sup.a      YT323   32.31.sup.aYT323     28.36.sup.a      Thaumatin                              28.44.sup.abYT152     17.33            YT152   25.17.sup.b______________________________________Sweet Aftertaste       Mouth Watering______________________________________YT323     22.51.sup.a      Thaumatin                              17.41.sup.aThaumatin 21.64.sup.a      YT323   16.85.sup.aYT152     15.85            YT152   10.28______________________________________Licorice Aftertaste______________________________________Thaumatin 22.82.sup.aYT323     21.56.sup.aYT152     13.69______________________________________ .sup.a Means are not significantly different at the 0.5 level of probability. .sup.b Means are not significantly different at the 0.5 level of probability. 
    
     
                       TABLE 10______________________________________Licorice               Mouth Coating______________________________________Thaumatin 16.26.sup.a      YT323   17.72.sup.aYT323     15.95.sup.a      Thaumatin                              15.72.sup.aTY152     9.58             YT152   10.59______________________________________Caramelize Lactose     Lactose Aftertaste______________________________________YT323     8.56.sup.a       YT323   7.67.sup.aThaumatin 7.67.sup.ab      Thaumatin                              6.85.sup.abYT152     4.58.sup.b       YT152   4.46.sup.b______________________________________Sour Aftertaste        Bitter Aftertaste______________________________________Thaumatin 7.39             YT152   9.69.sup.aYT152     3.74.sup.a       Thaumatin                              9.46.sup.aYT323     3.51.sup.a       YT323   5.49______________________________________ .sup.a Means are not significantly different at the 0.5 level of probability. .sup.b Means are not significantly different at the 0.5 level of probability. 
    
     
                       TABLE 11______________________________________Summary of Significant Differences.sup.1(90 μg/ml)      (145 μg/ml)YT152 Has/Is:      YT323 Has/Is:______________________________________Equal Sweetness    Equal SweetnessLess Lingering Taste              Less Sour AftertasteLess Sweet Aftertaste              Less Bitter AftertasteLess Mouth WateringLess Licorice FlavorLess Licorice AftertasteLess Mouth CoatingLess Sour Aftertaste______________________________________ .sup.1 All attributes listed were significantly different at p &lt; 0.05. 
    
     
                                           TABLE 12__________________________________________________________________________Sweetener Characteristics - Time IntensityYT152 (104 μg/ml)             Standard YT323 (70 μg/ml)                                     Standard(N = 13, Rep = 3) Error    (N = 15, Rep = 3)                                     Error__________________________________________________________________________A. Intensity at Expectoration                   A. Intensity at ExpectorationYT152      64.31.sup.a             3.63     Thaumatin                              67.6.sup.a                                     3.86Thaumatin  63.18.sup.a             3.98     YT323   64.9.sup.a                                     4.03B. Maximum Sweetness Intensity                   B. Maximum Sweetness IntensityThaumatin  73.23.sup.a             3.85     Thaumatin                              72.8.sup.a                                     3.92YT152      70.72.sup.a             3.82     YT323   70.5.sup.a                                     4.09C. Time to Maximum      C. Time to MaximumThaumatin  14.25  1.15     Thaumatin                              11.8.sup.b                                     0.65YT152      12.54  0.83     YT323   10.6.sup.b                                     0.56D. Total Duration       D. Total DurationThaumatin  40.72  4.01     Thaumatin                              36.5   2.56YT152      33.10  2.71     YT323   31.4   2.36__________________________________________________________________________ .sup.a Means are not significantly different at the 0.5 level of probability. .sup.b Means represent interpreted trends based on ranks and individual respondent data. 
    
     EXAMPLE 8 
     In this example, KHKl, a preferred yeast strain derived from PS16 was isolated. A PS16 [pING816A] transformant was streaked onto SD-ura agar for single colony isolation. Sixteen colonies were tested for thaumatin secretion by growth for four days in 3 mls of liquid SD -  ura. One of these, called BB2, which secreted 12.9 μg/ml of thaumatin, was sub-cloned by re-streaking onto SD -  ura agar. Four colonies were picked for growth in SD -  liquid. One of these, KHKl (originally BB2a) secreted 14-17 μg/ml. Yeast strain KHKl is on deposit under contract with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Maryland 20852-1776 as A.T.C.C. Deposit No. 20954. 
     Because of the XhoI site in the EcoRI fragment containing the uraA gene, the pING816A plasmid background was inconvenient for routine cloning of BamHI-XhoI fragments in front of the PGK terminator. Therefore, a similar plasmid background was constructed with this XhoI site destroyed. pING816A was cleaved with BamHI and XbaI, blunt-ended with T4 DNA polymerase, and ligated. This removed approximately 2900 bp, destroying the XbaI site and recreating the BamHI site. This intermediate plasmid, pKK135, was then cleaved with XhoI, blunt-ended with T4 DNA polymerase, and religated, thus destroying the XhoI site but creating a PvuI site. This second intermediate, pKK136, was cut with BamHI and HpaI and ligated with the approximately 2150 bp BamHI-HpaI fragment of pING58. The resulting plasmid, pING827, contains the betalactamase gene for ampicillin selection and the colEI replicon for replication in E. coli; the uraA gene from K. lactis and the leu2-d marker which can provide URA3 and partial LEU2 functions to S. cerevisiae as selectable markers; the origin of replication and STB (REP3) functions of the endogenous 2-micron plasmid for replication of pING827 in S. cerevisiae; and the PGK terminator behind a unique XhoI site. 
     Plasmid pING827 is suitable for cloning BamHI-XhoI fragments containing a promoter (in this case the PGK promoter) and a functional gene following the promoter (in this case a thaumatin gene having a secretion signal) in front of the PGK terminator. Plasmid pING834 was constructed by replacing the approximately 190 bp BamHI-XhoI fragment of pING827 with the approximately 1280 bp BamHI-XhoI fragment from pING460. This effectively created a version of pING816A with the XhoI site in uraA destroyed. Like pING816A, pING834 has the [Lys 46 , Asp 113  ] thaumatin I coding sequence. 
     A related plasmid, pING835, containing the [Lys 46 , Asp 113 , Asp 137  ] thaumatin I coding sequence can be constructed by ligation of three restriction fragments: The large BamHI-XhoI fragment of pING827; an approximately 540 bp BamHI-EcoRI fragment from pING460 containing the PGK promoter, the signal sequence, and the 5&#39; portion of the [Lys 46 , Asp 113  ] thaumatin I coding sequence; and an approximately 267 bp EcoRI-XhoI fragment containing the 3&#39; portion of the [Lys 46 , Asp 113 , Asp 137  ] thaumatin I coding sequence. This EcoRI-XhoI fragment can be supplied from pING152CVS, but was, in fact, supplied from another plasmid, pING152KRS. 
     KHKl [pING816A] was grown in small-scale fermenters where the level of secreted thaumatin present in the clarified culture media consistently exceeded 100 mg/liter. Specifically, seed cultures of PS-16 or KHKl with plasmid pING816A, pING834 or pING835 were grown at 30° C. in SD-ura with six times, i.e., 0.18 g/L, the normal amount of leucine. 5-50 mls of seed culture of OD 600  &gt;2 were inoculated aseptically into about one liter of a batch medium based on Fieschko, et al., Biotech. &amp; Bioeng., Vol. XXIX, pp. 1113-1121, John Wiley &amp; Sons (1987), produced by combining 90 parts by volume of a solution comprising 23.0 g/L casaminoacids, 4.0 g/L ammonium sulfate and 13.0 g/L potassium phosphate (monobasic) with 10 parts of separately autoclaved additions comprising Part 1 (2.0 g/L glucose, 1.0 g/L magnesium sulfate and 0.1 g/L inositol) and Part 2 (3.0 ml/L vitamin solution, 3.0 ml/L trace metals solution and 3.0 ml/L 1% thiamine solution). Although selection is maintained during growth of the seed, the exact method of seed growth is not considered crucial. The fermenter culture was grown at 30° C., while maintaining the pH at 5.5 with 40% ammonium hydroxide addition, and maintaining the dissolved oxygen with forced air and oxygen at greater than 35%. When the OD 600  reached approximately 1 in the fermenter, feed medium based on Fieschko, et al. and comprising 530 ml of Part 1 (100 g/L casamino acids, 3.0 g/L potassium phosphate (monobasic), 5.0 g/L ammonium sulfate, 0.5 ml/L polypropylene glycol and 465.0 ml/L RO (reverse osmosis)-H 2  O) and 455 ml of Part 2 (500.0 g/L glucose, 15.0 ml/L 1M magnesium sulfate, 0.1 g/L inositol and 120 ml/L RO-H 2  O) to which had been added after heating to dissolve the glucose 7.0 ml vitamin solution, 7.0 ml of trace metals solution and 2.0 mls 1% thiamine solution. The feed solution was added starting at a rate of about 1 ml/hour. The feed was stepped up proportionally to the OD 600  (determined by withdrawing and measuring samples from the fermenter) but, more importantly, the ethanol concentration (from yeast matabolism) was maintained at less than 0.1 g/L. The fermentation was continued for 2 to 8 hours after the feed had run out. The typical final OD 600  was greater than 200 but less than 250. 
     The values for thaumatin secreted were determined by an ELISA using thaumatin specific monoclonal and polyclonal antibodies. The amounts of thaumatin secreted into fermentation media were measured by at least one of two additional methods: (1) band intensity of desalted media run on SDS polyacrylamide gels stained with Coomassie blue; and (2) protein peak areas of media samples eluted using cation exchange chromatography (either HPLC or FPLC). For HPLC, a ten minute gradient of 0-500 mM NaCl in 10 mM Na-PO 4 , pH 6.9, over a BioRad HRLC MA7C column (carboxymethyl groups) was used. For FPLC, the same gradient in 20 mM MES [2-N-Morpholino) Ethanesulfonic Acid], pH 6.0, over a Rainin Hydropore-SCX column was used. According to this method of cation exchange chromatography, thaumatin elutes as a single peak while other media proteins fail to bind to the column in this chromatography scheme. 
     Numerous modifications and variations in practice of the invention are expected to occur to those skilled in the art upon consideration of the foregoing descriptions of preferred embodiments thereof. In particular, it is expected that in light of applicants&#39; discovery of the importance of the 97th and 137th amino acid residues to the taste characteristics of the thaumatin I molecule, that additional thaumatin I molecules having improved taste characteristics can be produced by substitution of amino acids other than aspartic acid for lysine at positions 97 and 137.