# Plant Synthetic Promoters
## 3 Objectives and General Methodology of Synthetic Promoter Creating
### Cis-Element Juggling and Domain Swapping  
As the components of synthetic promoters, existing juggled or shuffled cis-elements of oligonucleotides are usually involved. However, larger promoter domains with known regulatory properties could be exchanged in the process of domain swapping [215, 216, 217, 218, 219, 220]. Alternatively, completely novel cis-elements or domain variants identified by searches of available promoter databases or obtained through artificial intelligence support could be experimentally validated in plant material and then employed to construct synthetic promoters [221]. An example of cis-element shuffling involves the placement of the cis-elements of known functions in a novel or synthetic stretch of DNA. Optionally, fragments of one promoter could be exchanged with functionally equivalent domains from other heterologous promoters [195]. The latter approach is often practiced as ligating the upstream activation sequence (UAS) from one promoter to the TATA box-containing domain of another promoter. It is expected that the transfer of the cis-element or upstream DNA sequence from one promoter into another containing the TATA sequence could provide a novel transcription regulatory mechanism [128, 181, 222]. Placing the TATA-box of the CAMV35S promoter (TATATA) within a synthetic stretch of DNA formed a synthetic core promoter demonstrating 85% of the activity of the native sequence. However, core promoters prepared by domain swapping were significantly weaker, indicating only 20-68% of CAMV35S activity. These relatively weaker synthetic promoters could be important for complex metabolic engineering approaches, where the decreased or precisely tuned load of heterologously expressed genes could be salutary to host primary metabolic routes [195, 223, 224, 225, 226, 227]. Further applications of domain hybridization generated many superior plant promoters [128, 181, 209, 222, 228, 229, 230]. Viral and plant pathogen genomes are relatively simple and easily available sources of naturally occurring cis-active elements that can be used to prepare novel synthetic promoters through domain hybridization [181, 183, 222, 230]. These synthetic promoters could indicate the higher activity of CAMV35S. Indeed, two recombinant promoters, S100 and D100, prepared from the strawberry vein banding virus, demonstrated 1.8 and 2.2 times greater activity in tobacco (_Nicotiana tabacum_) than the CaMV35S promoter. The S100 recombinant promoter (621 bp) was obtained from the 250 bp long upstream activation sequence (UAS) of the strawberry vein banding virus (SV10 UAS; \(-\)352 to 102 relative to TSS) and the 371 bp long TATA-containing core promoter domain (SV10 CP; \(-\)352 to +19) of the same gene. Correspondingly, the 726 bplong D100 promoter was assembled by fusion of the 170 bp long UAS of the dahlia mosaic virus (DaMV14 UAS; \(-\)203 to \(-\)33) with its 556-bp-long core promoter domain (DaMV4 CP; \(-\)474 to +82) [231]. Similarly, the synthetic promoter FUASCsV8C was designed by placing the upstream activation sequence (UAS) of the figwort mosaic virus (FMV; \(-\)249 to \(-\)54) at the 5\({}^{\prime}\)-end of the cassava vein mosaic virus (CsVMV) promoter fragment 8 (CsVMV8; \(-\)215 to +166). This hybridized, synthetic promoter exhibited approximately 2.1-times higher _GUS_ activities in tobacco prototoplasts and 2.0-times higher activity in agroinfiltration assays, compared to the CaMV35S promoter [222]. Synthetic promoters can be significantly stronger than CAMV35, as shown by the construction of two chimeric promoters, MBR3 and FBR3, by fusing the UASs (upstream activation sequences) of mirabilis mosaic virus(MUAS; \(-\)297 to +38; 335 bp) and figwort mosaic virus(FUAS; \(-\)249 to +54; 303 bp), respectively, to the core promoter domain of BR3(BR3; \(-212\) to +160; 372 bp). The activities of MBR3 and FBR3 promoters were comparable to the activity of the CaMV35S2 promoter and four times stronger than that of the CaMV35S, as confirmed by histochemical and fluorometric GUS assays [232].