Abstract:
An object of the present invention is to provide a means for producing an archaeal protein by extracellular secretion. The system for secretory production of an archaeal protein uses a fungus belonging to the genus  Talaromyces  as a host.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of International Application No. PCT/JP2015/061068, filed on Apr. 9, 2015, which claims priority to Japanese Application No. 2014-081115, filed on Apr. 10, 2014. The contents of both applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    Technical Field 
         [0003]    The invention disclosed herein partly relates to a system for secretory production of archaeal proteins. 
         [0004]    Background Art 
         [0005]    Archaea are known to produce various useful proteins. For example, thermostable cellulases, which are useful for saccharification of biomass, have been identified from thermophilic archaea or hyperthermophilic archaea (i.e.,  Pyrococcus furiosus, Pyrococcus horikoshii, Thermotoga maritima, Sulfolobus shibatae , and  Sulfolobus solfataricus ). 
         [0006]    As a means for industrially producing archaeal proteins, for example, a method for producing hyperthermostable endo-cellulases of  Pyrococcus horikoshii  in  Escherichia coli  has been reported (Non-patent literature (NPL) 1). A method for producing hyperthermostable endo-cellulases of  Pyrococcus furiosus  using  Escherichia coli  as a host has also been reported (Non-patent Literature (NPL) 2). 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: JP2008-271927 
       
     
       Non-Patent Literature 
       [0000]    
       
         NPL 1: Ando et al., Appl. Environ. Microbiol., 68, 1, 430-433, 2002 
         NPL 2: Bauer et al., J. Bacteriol., 181, 1, 284-290 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0010]    However, previously reported methods could still be further improved. For example, the methods disclosed in NPL 1 and NPL 2 require cellular disruption to obtain a protein since these methods are substantially not capable of producing archaeal proteins by extracellular secretion. In view of this, an object of the present invention is to provide a means for producing archaeal proteins by extracellular secretion. 
       Solution to Problem 
       [0011]    There has been no report regarding the expression of archaeal proteins by secretion even when  Trichoderma reesei , which is a typical filamentous fungus for high amounts of protein production by expression, is used as a host. Regarding archaeal beta-glucosidase, there is a single report stating that the protein is produced by expression using  Trichoderma reesei  as a host; however, secretion could not be achieved. In view of this, the present inventors conducted extensive research to solve the above problems, and found that the use of filamentous fungi of the genus  Talaromyces  as a host enables the production by secretion. The present invention has been accomplished as a result of further research and improvements based on these findings. 
         [0012]    Representative examples of the present invention are described below. 
       Item 1. 
       [0013]    A system for secretory production of an archaeal protein, the system using a fungus belonging to the genus  Talaromyces  as a host. 
       Item 2. 
       [0014]    The system according to Item 1, wherein the fungus belonging to the genus  Talaromyces  is transformed with an expression vector comprising: 
         [0015]    a promoter region that functions in the fungus belonging to the genus  Talaromyces,    
         [0016]    a secretion signal region located downstream of the promoter region, 
         [0017]    an archaeal protein-coding region located downstream of the secretion signal region, and 
         [0018]    a terminator region located downstream of the archaeal protein-coding region. 
       Item 3. 
       [0019]    The system according to Item 2, wherein the secretion signal region has the base sequence of SEQ ID NO: 5. 
       Item 4. 
       [0020]    The system according to Item 2 or 3, wherein both the promoter region and the terminator region are of glucoamylase from  Talaromyces cellulolyticus.    
       Item 5. 
       [0021]    The system according to any one of Items 2 to 4, wherein the codon usage in the archaeal protein-coding region is optimized for expression in a fungus belonging to the genus  Talaromyces.    
       Item 6. 
       [0022]    The system according to any one of Items 1 to 5, wherein the archaeal protein is a cellulase from the genus  Pyrococcus.    
       Item 7. 
       [0023]    The system according to any one of Items 2 to 6, wherein the archaeal protein-coding region has the base sequence of SEQ ID NO: 6 or 7. 
       Item 8. 
       [0024]    A method for producing an archaeal protein by secretion using the system of any one of Items 1 to 7. 
       Item 9. 
       [0025]    The method according to Item 8, wherein the method comprises a step of culturing a fungus belonging to the genus  Talaromyces.    
       Item 10. 
       [0026]    Use of a fungus belonging to the genus  Talaromyces  in the secretory production of an archaeal protein. 
       Advantageous Effects of Invention 
       [0027]    The present invention enables efficient secretory production of archaeal proteins. Thus, the present invention makes it possible to industrially and stably provide archaeal proteins. According to the present invention, an archaeal protein can be obtained without crushing a microorganism that produces the protein. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  shows the structure of a vector constructed in the Examples. 
           [0029]      FIG. 2  shows the molecular weight determined by SDS-PAGE analysis of a thermostable endo-cellulase from  Pyrococcus furiosus  and thermostable endo-cellulase from  Pyrococcus horikoshii  recombinantly produced by secretion in the Examples using a fungus of  Talaromyces  as a host. Lanes 1 and 6: molecular markers; Lane 2: a sample obtained from a culture medium in which the thermostable endo-cellulase from  Pyrococcus furiosus  was produced by secretion; Lane 3: a sample obtained by heating a culture medium in which the thermostable endo-cellulase from  Pyrococcus furiosus  was produced by secretion, the heating being performed at 70° C. for 15 minutes to remove contaminating proteins; Lane 4: a sample obtained from a culture medium in which the thermostable endo-cellulase from  Pyrococcus horikoshii  was produced by secretion; and Lane 5: a sample obtained by heating a culture medium in which the thermostable endo-cellulase from  Pyrococcus horikoshii  was produced by secretion, the heating being performed at 70° C. for 15 minutes to remove contaminating proteins. 
           [0030]      FIG. 3  shows the results of the endoglucanase activity measured at 50° C., 60° C., 70° C., and 80° C., with respect to the thermostable endo-cellulase from  Pyrococcus furiosus  recombinantly produced by secretion using a fungus of the genus  Talaromyces  as a host. 
           [0031]      FIG. 4  shows the results of the endoglucanase activity measured at 50° C., 60° C., 70° C., and 80° C., with respect to the thermostable endo-cellulase from  Pyrococcus horikoshii  recombinantly produced by secretion using a fungus of the genus  Talaromyces  as a host. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    In secretory production of archaeal proteins, it is preferable to use fungi belonging to the genus  Talaromyces  as a host. The fungi belonging to the genus  Talaromyces  as used herein are a species of filamentous fungi and are used in the industry to produce antibiotics and various enzymes. The fungi belonging to the genus  Talaromyces  are not particularly limited as long as secretory production of archaeal proteins is possible; it is preferable to use fungi that themselves have an ability to produce proteins (preferably cellulases) by secretion. Examples of such fungi belonging to the genus  Talaromyces  include  Talaromyces cellulolyticus, Talaromyces flavus, Talaromyces marneffei, Talaromyces pinophilus, Talaromyces emersonii, Talaromyces stipitatus , and the like. As  Talaromyces cellulolyticus , it is preferable to use Y-94, YP-4, TN, C-1, and CF-2612 strains. Until recently,  Talaromyces cellulolyticus  had been categorized as  Acremonium cellulolyticus.    
         [0033]    The archaeal protein produced by secretion using fungi belonging to the genus  Talaromyces  as a host may be any archaeal protein and may be appropriately selected according to the purpose. Examples of archaeal proteins include industrially used various enzymes (e.g., hydrolase, oxidoreductase, transferase, isomerase, synthetase, ligase, and lyase). More specific examples include cellulase, xylanase, amylase, lipase, protease, xylanase, pectinase, chitinase, kinase, luciferase, and the like. In one embodiment, a preferable enzyme is a cellulase, such as endo-cellulase, exo-cellulase (cellobiohydrolase), and β-glucosidase. In one embodiment, it is preferable that the cellulase is a monomeric protein. It is also preferable that the cellulase belongs to glycosyl hydrolase family 5 or 12 because most of the cellulases in these families are monomeric cellulases. 
         [0034]    In one embodiment, a protein produced by secretion is preferably an endo-cellulase. An endo-cellulase refers to an enzyme that catalyzes a reaction for hydrolysis of β-glycosidic linkages in amorphous cellulose molecules. The activity thereof can be evaluated mainly using carboxymethyl cellulose (CMC) as a substrate. For example, the activity is determined in the following manner: 50 μL of an enzyme dilution is added to 950 μL of 50 mM acetic acid buffer containing 1% CMC; then, after 30 minutes, the reaction is stopped by adding 3 mL of a strong alkaline DNS assay solution, and the amount of reducing ends released from CMC is quantified by the DNS method. One U is defined as the activity in which 1 μmol of reducing sugars are generated per minute. 
         [0035]    The type of archaea, which is the origin of the proteins produced by secretion, may be any archaea and may be suitably selected according to the type of proteins to be produced. In one embodiment, it is preferable to use archaea classified into, for example, hyperthermophilic, halophilic, or acidophilic archaea. Preferable examples of hyperthermophilic archaea include archaea belonging to the genera  Pyrococcus, Thermococcus, Thermotoga , and  Sulfolobus . More specific examples include  Pyrococcus horikoshii, Pyrococcus furiosus, Thermococcus kodakaraensis, Thermotoga maritima, Sulfolobus shibatae , and  Sulfolobus solfataricus.    
         [0036]    In one embodiment, proteins produced by secretion are preferably thermostable endo-cellulases from the genus  Pyrococcus . More specifically, a thermostable endo-cellulase from  Pyrococcus horikoshii  and a thermostable endo-cellulase from  Pyrococcus furiosus  are preferable. The amino acid sequence of the thermostable endo-cellulase from  Pyrococcus horikoshii  and a base sequence encoding this amino acid sequence are respectively shown in SEQ ID NOs: 1 and 2. The amino acid sequence of the thermostable endo-cellulase from  Pyrococcus furiosus  and a base sequence encoding this amino acid sequence are respectively shown in SEQ ID NOs: 3 and 4. The thermostable endo-cellulase from  Pyrococcus furiosus  and thermostable endo-cellulase from  Pyrococcus horikoshii  are stable even in the temperature range of 80° C. or higher, and show high activity. Their use (in particular in saccharification of biomass) is thus promising. 
         [0037]    Archaeal proteins may be found in nature or may be artificially modified. For example, it is possible to use a protein having an amino acid sequence with 80% or more, 85% or more, 90% or more, 95% or more, or 98% or more identity to an amino acid sequence of known proteins (e.g., the amino acid sequence shown in SEQ ID NO: 1 or 3), and having endo-cellulase activity. 
         [0038]    In the amino acid sequence of SEQ ID NO: 1, the amino acid region from position 1 to position 28 is a signal sequence, the amino acid region from position 29 to position 416 is an active domain, and the region after position 417 is a functionally unknown membrane-binding region. In the amino acid sequence of SEQ ID NO: 3, the amino acid region from position 1 to position 18 is a signal sequence, the amino acid region from position 19 to position 49 is a functionally unknown region, and the region after position 50 is an active domain. 
         [0039]    It is preferable that the fungi belonging to the genus  Talaromyces  for the secretory production of archaeal proteins is transformed with a vector incorporating an archaeal protein-encoding base sequence in a manner that allows for the expression. Such a vector preferably comprises a promoter region, a secretion signal region located downstream of the promoter region, an archaeal protein-coding region located downstream of the secretion signal region, and a terminator region located downstream of the archaeal protein-coding region, all of which regions function in fungi belonging to the genus  Talaromyces.    
         [0040]    The promoter region and terminator region that function in fungi belonging to the genus  Talaromyces  are not particularly limited, and may be suitably modified and selected. For example, they may be suitably selected from promoter and terminator regions of filamentous fungi, and preferably selected from promoter and terminator regions of the genus  Talaromyces . A plurality of promoter and terminator regions from the genus  Talaromyces  have already been known. Further, novel promoter and terminator regions are easily searchable based on well-known promoter and terminator base sequences, or well-known protein sequences, using known techniques. The origin of the promoter and terminator regions may be fungi belonging to the genus  Talaromyces  to be used as a host, or may be other fungi. Examples of suitable promoters and terminators from fungi belonging to the genus  Talaromyces  include starch-inducible glucoamylase promoter and glucoamylase terminator, which are used in the Examples described below. Additionally, promoters and terminators that control the expression of Cellobiohydrolase I present in a fungus belonging to the genus  Talaromyces  are also suitably used. They can more strongly express genes by adjusting culture conditions. 
         [0041]    The secretion signal region is not particularly limited as long as it allows archaeal proteins to be extracellularly secreted from fungi belonging to the genus  Talaromyces , and any modification and selection can be made. For example, it is possible to use signal regions of archaea. The origin of the signal region may be the same or different from that of proteins to be produced by secretion; the signal region may be one naturally present in the proteins to be produced by secretion. Specific examples of preferred signal regions include the region encoding a secretion signal peptide of Cellobiohydrolase from  Talaromyces cellulolyticus  shown in SEQ ID NO: 5. 
         [0042]    The regions (or base sequences) encoding a secretion signal and an archaeal protein are not particularly limited as long as the expression in a fungus belonging to the genus  Talaromyces  is possible. It is preferable that the codon usage is optimized for suitable expression in a fungus belonging to the genus  Talaromyces . The codon usage may be optimized with respect to a part or all the minor codons in the base sequences encoding proteins; however, it is preferable that as many minor codons as possible are optimized. The term “minor codon” as used herein refers to codons other than most frequently used codons (major codons) among those coding for specific amino acids in a fungus belonging to the genus  Talaromyces . Optimization of codon usage is desirably performed to such an extent that the optimization does not change the amino acids encoded by the codons. The technique for codon usage optimization is known, and any means may be used. For example, optimization may be performed by introducing mutations using a nucleic acid primer or by nucleic acid synthesis. 
         [0043]    The optimization of the codon usage to increase the amount of expression in a fungus belonging to the genus  Talaromyces  may be performed by changing codons encoding each amino acid to the following: phenylalanine (UUC), leucine (CUC), isoleucine (AUC), valine (GUC), serine (UCU), proline (CCU), threonine (ACC), alanine (GCU), tyrosine (UAC), histidine (CAU), glutamine (CAA), asparagine (AAC), lysine (AAG), aspartic acid (GAU), glutamic acid (GAA), cysteine (UGC), arginine (CGA), and glycine (GGC). SEQ ID NO: 6 shows the base sequence encoding thermostable endo-cellulase from  Pyrococcus horikoshii , the codon usage of which sequence has been optimized for the expression in the genus  Talaromyces . SEQ ID NO: 8 shows the base sequence encoding thermostable endo-cellulase from  Pyrococcus furiosus , the codon usage of which sequence has been optimized for the expression in the genus  Talaromyces.    
         [0044]    The expression vector containing a promoter region, a secretion signal region, a protein-coding region, and a terminator region in a manner that allows for protein expression may be linked via an appropriate restriction enzyme recognition site. The expression vector may also contain a selectable marker gene, such as a drug resistance gene and an auxotrophic complementary gene. Such an expression vector may be constructed by using known methods. 
         [0045]    Known methods may be used to introduce an expression vector into a fungus belonging to the genus  Talaromyces  used as a host. For example, an electroporation method, a polyethylene glycol method, an  Agrobacterium  method, or the like may be used. The vector to be introduced may be a single vector, or two or more types of vectors. The recombination of the gene of a fungus of the genus  Talaromyces  using the introduced gene may be performed by homologous recombination or non-homologous recombination. To achieve an increased amount of expression by making use of homologous recombination, it is preferable to perform homologous recombination at the site of cellulase gene of a fungus belonging to the genus  Talaromyces . In particular, homologous recombination at the site of cellobiohydrolase gene of a fungus belonging to the genus  Talaromyces  is preferable to achieve a high amount of enzyme expression. Further, in homologous recombination at the site of endo-cellulase gene of a fungus belonging to the genus  Talaromyces , the cellobiohydrolase activity is not decreased, and the biomass decomposition activity is thus less likely to be reduced, compared with homologous recombination at the site of a cellobiohydrolase gene; thus, homologous recombination at the site of endo-cellulase gene of a fungus belonging to the genus  Talaromyces  is preferable. In all of these cases, homologous recombination is preferably performed at a site that is linked to a promoter, signal peptide, terminator, and the like, of target cellobiohydrolase and/or endoglucanase genes so that the promoter and terminator of endogenous cellobiohydrolase gene and/or endo-cellulase gene of a fungus belonging to the genus  Talaromyces  are used as is. 
         [0046]    For non-homologous recombination, the recombination target genes are not particularly limited. In non-homologous recombination, the loss of cellulase gene rarely occurs, and the biomass decomposition activity is thus less likely to be reduced; therefore, non-homologous recombination is preferable. When non-homologous recombination is performed, a gene to which a promoter and a terminator are suitably linked is used to express a target protein. 
         [0047]    When the thus transformed host is cultured, a target archaeal protein is expressed and extracellularly secreted. Culture of a transformant is performed in accordance with known methods. As a medium, for example, commonly used components may be used. For example, a carbon source may be used, such as glucose, sucrose, cellulose, sugar syrup, dextrin, starch, glycerol, molasses, and animal and plant oils. Further, a nitrogen source may be used, such as polypeptone, soybean flour, wheat germ, cotton seed meal, bouillon, peptone, yeast extract, ammonium sulfate, potassium nitrate, and urea. Additionally, it is also effective to add, as required, inorganic salts, such as potassium chloride, magnesium sulfate, monopotassium phosphate, zinc sulfate, manganese sulfate, and copper sulfate, that can generate ions of sodium, potassium, calcium, magnesium, cobalt, chlorine, phosphate, and sulfate, as well as other ions. It is also possible to add micronutrients, such as various vitamins, amino acids, and nucleotides, as well as selection drugs, such as antibiotics, as required. In addition, it is possible to suitably add organic substances and inorganic substances that help cell growth and accelerate the expression of introduced genes. Examples of such substances include cellulose, xylan, lactose, and the like. 
         [0048]    The culture method may be arbitrarily selected, and various culture systems, such as shaking culture, spinner culture, agitation culture, stationary culture, and continuous culture, may be used, with shaking culture or spinner culture being preferable. The culture temperature is usually 20 to 35° C., and preferably 25 to 31° C. The culture time may be suitably set according to the amount of target protein to be produced by secretion, and is, for example, 3 to 10 days, and preferably 4 to 9 days. 
         [0049]    The target protein secreted in the culture medium by culturing the host may be used as is as the culture medium. It is also possible to concentrate, extract, and/or purify the culture medium to obtain a concentrate, an extract, a roughly purified protein, or a purified protein. Any techniques may be used for concentration, extraction, and purification. For example, techniques such as filtration, centrifugation, salting-out, solvent precipitation, dialysis, ultrafiltration, gel electrophoresis, and various chromatographies (e.g., ion exchange, hydrophobic, reverse phase, gel filtration, affinity) may be used in combination. 
       EXAMPLES 
       [0050]    The present invention is described in more detail with reference to Examples but is not limited to these. 
       1. Preparation of Recombinant Vector 
       [0051]    A polynucleotide (hereinafter sometimes also referred to as “sequence H”) having a base sequence in which the base sequence (SEQ ID NO: 6) encoding a thermostable endo-cellulase from  Pyrococcus horikoshii , the codons of whose sequence had been modified for the expression in the genus  Talaromyces , was linked to the downstream of the base sequence (SEQ ID NO: 5) encoding the secretion signal peptide of cellobiohydrolase from  Talaromyces cellulolyticus  was prepared by a synthetic method. Further, a polynucleotide (hereinafter sometimes also referred to as “sequence F”) having a base sequence in which the base sequence (SEQ ID NO: 7) encoding a thermostable endo-cellulase from  Pyrococcus furiosus , the codons of whose sequence had been modified for the expression in the genus  Talaromyces , was linked to the downstream of the base sequence (SEQ ID NO: 5) encoding the secretion signal peptide of cellobiohydrolase from  Talaromyces cellulolyticus  was prepared by a synthetic method. These two types of polynucleotides were incorporated into plasmid vectors (Inoue et al., J. Ind. Microbiol. Biotechnol., 2013, 40: 823-830) containing starch-inducible glucoamylase promoter and glucoamylase terminator of  Talaromyces cellulolyticus . In this manner, a plasmid vector having the sequence H between the glucoamylase promoter and glucoamylase terminator (hereinafter sometimes referred to as “the vector H”) and a plasmid vector having the sequence F between the glucoamylase promoter and glucoamylase terminator (hereinafter sometimes referred to as “the vector F”) were produced ( FIG. 1 ). As shown in  FIG. 1 , restriction enzyme recognition sites (ECORV site and Sbfl site) were appropriately provided at the linkages between each region. 
       2. Production of Transformed  Talaromyces    
       [0052]    The recombination vectors prepared in 1. above were separately introduced into  Talaromyces cellulolyticus  YP-4. The introduction was performed by a protoplast PEG method, and the transformants were selected using a medium for uracil-auxotrophic selection. The following is the composition of the medium for uracil-auxotrophic selection: 
         [0000]    1 mass % glucose,
 
1.0 M sucrose,
 
1.5 mass % agar,
 
0.04 mass % KH 2 PO 4  (pH 5.5),
 
0.01 mass % potassium chloride,
 
0.01 mass % MgSO 4 .7H 2 O,
 
0.05 mass % ammonium chloride, and
 
0.5 mass % trace element.
 
The trace element was obtained by dissolving 200 mg of CuSO 4 .5H 2 O, 200 mg of MnCl 2 .5H 2 O, and 200 mg of ZnSO 4 .7H 2 O in distilled water, and filling up to 100 mL. A PCR method was used to confirm the insertion of the target gene into the genomic DNAs of the obtained transformants.
 
       3. Secretory Production of Archaeal Protein Using Transformed  Talaromyces    
       [0053]    Each transformant was separately inoculated into a 100-mL Erlenmeyer flask containing 10 mL of liquid medium, and cultured at 220 rpm for four days at a pH of 4.0 at 28.5° C. The composition of the medium was as follows: 
         [0000]    1 mass % corn steep liquor,
 
2 mass % starch,
 
0.5 mass % (NH 4 ) 2 SO 4 ,
 
2.4 mass % KH 2 PO 4 ,
 
0.47 mass % potassium tartrate dihydrate,
 
0.12 mass % MgSO 4 ,
 
0.1 mass % Tween 80,
 
0.2 mass % urea, and
 
0.1 mass % Trace element.
 
Each of the obtained culture media was subjected to centrifugal separation at 3500 rpm for 10 minutes, and separated into a culture supernatant and cells. The culture supernatant was subjected to heat treatment at 70° C. for 15 minutes, followed by the measurement of cellulase activity to confirm its presence. The protein amounts of thermostable endo-cellulase from  Pyrococcus horikoshii , and thermostable endo-cellulase from  Pyrococcus furiosus  contained in the culture supernatants after heat treatment were measured by a BCA method, both of which resulted in the expression amount of 100 mg/L or more, on a BSA basis.
 
       4. Measurement of Molecular Weight 
       [0054]    The culture supernatants obtained in 3. above were subjected to heat-treatment at 70° C. for 15 minutes, and contaminating proteins were removed, followed by the measurement of the molecular weight by SDS-PAGE. The molecular weights of thermostable endo-cellulase from  Pyrococcus horikoshii  and thermostable endo-cellulase from  Pyrococcus furiosus  were 44 kDa and 30 kDa, respectively, and they were thus confirmed to be target enzymes ( FIG. 2 ). 
       5. Measurement of Thermostable Endo-Cellulase Activity 
       [0055]    The thermostable endo-cellulase from  Pyrococcus horikoshii  and thermostable endo-cellulase from  Pyrococcus furiosus  expressed in the filamentous fungi of the genus  Talaromyces  were purified, and the endoglucanase activity was measured at reaction temperatures of 50, 60, 70, and, 80° C. Specifically, 3.0 μg of thermostable endo-cellulase from  Pyrococcus horikoshii  or 1.5 μg of thermostable endo-cellulase from  Pyrococcus furiosus  was added per 1 mL of 20 mM sodium acetate buffer containing CMC in an amount of 1 mass %, and reacted for 1 hour at each temperature. The amount of generated reducing ends was measured by DNS method to determine the activity. The results confirmed a thermal resistance as high as that of wild-type endoglucanase. These enzymes were thus confirmed to have thermal resistance ( FIGS. 3 and 4 ). In  FIGS. 3 and 4 , the horizontal axis represents the measurement temperature, while the longitudinal axis represents the activity at each temperature, relative to the activity at 50° C., which is considered to be 100%.