Team:NUDT CHINA/Improvement
Improvement
1.1 Original Part: TRE-3G promoter
TRE-3G promoter(PTRE3G, BBa_K1431301) is a kind of eukaryocyte cell promoters that will be switched on upon Tet-On 3G binding under the existence of doxycycline(Dox).
1.2 Our Improvement
Our improved part is TCE promoter (BBa_K4016011), a tetO7 promoter consists of seven direct 19-bp tet operator sequence (tetO) repeats upstream of a minimal CMV promoter (PminCMV). tetOs were separated by a 17bp flanking sequence, which was essential to the dox responsiveness of this promoter. The tetO constitutively binds tetR, it also binds to transcriptional activator tet-on 3G under the presence of doxycycline. This promoter should be constructed just upstream of a Kozark consensus ribosome binding site to enhance the expression level of GOI.
The improved TCE promoter shares a similar nucleic acid sequence to the original promoter in BBa_K1431301, with only a few nucleotides different on the flanking regions of tetO. However, these differences were sufficient to generate folds of changes on dox responsiveness.
We used SEAP assay to validate the improvement. SEAP reporter gene was constructed downstream of original TRE3G promoter and TCE promoter as BBa_K1431301-SEAP and BBa_K4016011-SEAP. Therefore the SEAP activity in the culture medium could be used to report the activation of the promoters.
Meanwhile, we constructed tetR-3G into pcDNA3.1 vector to obtain pcDNA3.1-tetR-3G. TetR-3G can specifically bind to tetO and turn on the transcription of its downstream genes SEAP. SEAP activity could be measured witha para-Nitrophenylphosphate (pNPP) based assay, as SEAP catalyzes the reaction from pNPP to para-Nitrophenol, this new product absorbs light at 405 nm and can be measured via photometry.
Either BBa_K1431301-SEAP encoding plasmid or BBa_K4016011-SEAP encoding plasmid were co-transfected with tetR-3G encoding plasmid into HEK-293 cells. Cells were cultured for 6h before adding 500 ng/ml doxycycline, cells transfected with the same plasmids without doxycycline treatment were used as control. SEAP activity was detected 24/48 hours after transfection.
HEK-293T cells co-transfected with tetR-3G expressing plasmid and either BBa_K4016011-SEAP or BBa_K1431301-SEAP plasmid showed significantly boosted SEAP production upon 24h or 48 h of doxycycline induction comparing to their respective control group, in which SEAP activities were below detection limit. This proves that both promoters did initiate the transcription of downstream gene. Intriguingly, under doxycycline induction, HEK-293T cells co-transfected with BBa_K4016011-SEAP and tetR-3G expressing plasmids showed ~30 times higher SEAP activity comparing to cells transfected with same dose of BBa_K1431301-SEAP and tetR-3G plasmids, showing a significantly improved performance of BBa_K4016011 comparing to BBa_K1431301.
Figure 1. Comparation of doxycycline responsiveness between the improved version and the original version of tetO7 promoters. (A) SEAP activity in the culture medium collect from cells 24 hours after 500 ng/ml doxycycline induction. (B) SEAP activity in the culture medium collect from cells 48 hours after 500 ng/ml doxycycline induction. BDL stands for below detection limit. Data represent mean ± s.e.m. (n=3 biological replicates)