Strains, plasmids, and culture conditions
Mucor rouxii DSM1194 was used as the source of delta-6 desaturase gene. The Escherichia coli DH5α (Stratagene. The USA) and pTZ57R/T (Invitrogen) were used as the host–vector system. Pichia pastoris host strain GS115 (Invitrogen. The UK) and pPICZC (Invitrogen) were used for protein expression. All chemicals were obtained from Merck with analytical grade and the restaurant oil wastes sample were collected from Naz plant oil factory.
Cultures were grown in Luria Bertani Agar (LB Agar) [20]. Media were supplemented with zeocine (25 μg/ml) when required. The pH was adjusted to 7.5 and 7 for mycelia and colonial growth, respectively.
Preparation of oil wastes media was carried out according to Papanikolaou method contained (per liter of distilled water) 7.0 g KH2PO4, 2.5 g Na2HPO4, 1.5 g MgSO4, 0.06 g MnSO4, 0.15 g CaCl2, 0.15 g FeCl3, 0.5 g yeast extract, 0.5 g (NH4)2SO4, pH 6.0, and 2% (V/V) oil waste as a carbon source [21].
In vitro assays of delta-6 desaturase gene existence
To verify that the enzyme gene existed in strains of M.rouxii and to ensure that mutations were not formed, DNA alignment was carried out. To this end, DNA extraction, PCR with special primers, and DNA sequencing were done using the Yamada et al [22], White et al, and Sanger methods, respectively. The primers used for the PCR are described below.
F primer: 5`-CAAGAATTCAAAATGGCTCCCCCAAATACTGCGGC3'
R primer: 5'-CAACTCGAGTTCTTTATCATTAGCCCAAATCTC'3
Total RNA isolation
The M.rouxii strain was grown at 28 °C in a 250 mL shaker flask containing 50 mL liquid medium in a shaking incubator (250 rpm). After a 48 h growth period, the strains were harvested and the total RNA from M.rouxii was isolated as described below.
Cells of the digestive gland were disrupted with a sterile homogenizer with liquid nitrogen. Total RNA was extracted using the RNAX-PLUS™ Total RNA Extraction Kit (CinnaGen) according to the manufacturer's instructions.
cDNA synthesis and PCR amplification
The partial cDNA fragment of M.rouxii delta-6 desaturase was amplified by RT-PCR. The primers used were Random Hexamer Primers, which were designed according to all kinds of cellular RNAs.
The reactions were performed using RevertAid M-MuLV Reverse Transcriptase. A 10 µL reaction containing 4 µL 5× PCR buffer, 1 µL Reverse Transcriptase, 2 µL of 10 mM dNTPs, 1 µL of primers, and 1 µg of the RNA sample was used. After mixing and centrifuge for 5 sec, samples were incubated at 65°C for 5min and 42°C for 1h. Finally, for deactivation of Reverse Transcriptase, samples were incubated at 70°C for 10 min. RT-PCR was performed on the cDNA product using GAPDH primers as a control.
Cloning of PCR product and transformation
The PCR products were characterized by agarose gel electrophoresis and extracted using a DNA extraction kit (Fermentase). Purified products were ligated into pTZ57R/T vector according to the manufacturer's instructions and transformed using the heat shock method into E. coli DH5α competent cells prepared by chemical CaCl2 method [20]. Colony PCR was carried out to verify that the Plasmid DNA had transformed correctly using the below primers.
F primer: 5`-CAAGAATTCAAAATGGCTCCCCCAAATACTGCGGC3'
R primer: 5'-CAACTCGAGTTCTTTATCATTAGCCCAAATCTC'3
Construction of the expression plasmid
Plasmid DNA was purified using the plasmid extraction kit (#K0502; Fermentase) and digested using XhoI and EcoRI (Fermentase). Furthermore, pPICZC vector was also digested using XhoI and EcoRI, respectively and after incubation with Alkaline phosphatase (Fermentase), the digested plasmid was cleaned up. Digested pPICZC vector and delta-6 desaturase gene were ligated using DNA T4 ligase (#EL0014; Fermentase).
The ligation product was transformed into E. coli DH5α competent cells. The E. coli transformants were selected on plates containing the antibiotic, Zeocin™ at a concentration of 25 µg/ml. Verification of insertion of the PCR fragment into the correct translational reading frame was confirmed by colony PCR, double digestion, and DNA sequencing before the introduction of the chimeric plasmid into P. pastoris host cells. Sequencing primers (5′AOX1 and 3′AOX1) were obtained from Invitrogen.
Transformation of P. pastoris and expression in shaken flasks
P. pastoris GS115 strain was selected to be used as a host strain. The recombinant plasmid pPICZA-delta-6 desaturase was purified from E. coli cells and linearized with the restriction enzyme SacI to allow integration of the vector DNA into the chromosomal DNA.
Pichia transformation was performed using the electroporation (Eppendorf), and the high-level expression transformants were screened in the YPD (1% yeast extract, 2% peptone, 2% dextrose, 1.8% agar) plates supplemented with Zeocin™ (100 µg·mL−1).
For the screening of strains and for the optimization of culture conditions in shaking flasks, methanol 0.5% was used for induction, according to the Invitrogen instructions. GLA‐producing P. pastoris cells were grown in 50ml YPD with methanol 0.5% and linoleic acid 2% at 30°C for 4 days.
In vitro assays of GLA production
To verify that the delta-6 desaturase had been expressed and to ensure its activity, Sudan black and Nile red staining were carried out. For that experiment, samples were obtained from production media after 72 h and stained.
We also evaluated the amount of GLA production using flow cytometry on recombinant and wild-type species as a control. First, the FL2 channel of florescent recombinant P. pastoris lipid production was compared with wild type. Subsequently, SSC dot blots of the recombinant P. pastoris along with FL2 studies (FL2=1, SSC=10) against florescent were studied.
GLA purification and GC analysis
The modified Bligh and Dyer method was performed to extract lipid from P. pastoris cells [23]. After passing P. pastoris cells through Whatman No.1 filter paper, mycelia were washed by distilled water three times and once by ethanol, respectively. Samples were incubated at 60ºC for 2 h with HCl solution (10 mL of 4 M), and. The hydrolyzed solution was shaken with 20 mL of chloroform/methanol (1:1) at room temperature for 3 hours and centrifuged at 2000 ×g for 5 min. After the supernatant phase was separated, the lower phase containing lipids was evaporated under reduced pressure for 10 min. The extracted fatty acids were modified to fatty acid methyl esters (FAMEs) according to the Christie method [24].
The Gas Chromatography (GC) was performed using Agilent 19091J-413 Series with an FID and the capillary column DB-23 (USA). The injector and detector temperatures were maintained at 260 and 300°C, and the oven program was 100°C: 2 min; 160°C: 3 min; 215°C: 2 min; 217°C: 2 min; 218°C: 2 min, and 260°C for 2 min. The flow rate of nitrogen as the carrier gas was 1.5 mL/min.
Optimization in fermenter by using of oil waste
Under optimal conditions, recombinant yeast was cultured in a 3-liter fermenter (with 1.5 liters of culture fluid) and the results of lipid production and dry weight, and GLA production were obtained during 5 days of growth. It should be noted that oil waste was used as a carbon source and the experiment was done three times.