## 5 Transcription Factor Decoys Approach  
Transcription factor decoys (TFDs) are short double-stranded DNA molecules that mimic the specific binding site in the promoter of a transcription factor. They can interfere with gene expression by aberrant binding between the transcription factor and the target promoter. When the copy number of the TFDs is large, the repressor or activator only binds with the TFDs. That leads to decreased binding of the repressor or activator with the promoter. The results are de-repression or de-activation of the target gene by the TFDs. The TFD strategy has been widely applied in mammalian cells and preclinical studies [61; 62].  
However, the TFD strategy was used for the first time to activate the cryptic BGCs in _Streptomyces_[63]. As it is difficult to exactly predict the regulatory DNA fragments in the BGCs, DNA fragments of approximately 100 base pairs were cloned to both the upstream and downstream of open reading frames (ORFs) in the target BGC. These DNA fragments, the TFDs, were transferred into the native hosts. For proof of concept, they activated two silent BGCs of actinorhodin and undecylprodigiosin in _S. lividans_ 66 by using the low-throughput TFD strategy. Five putative regulators in the BGC of actinorhodin and three putative regulators in the BGC of undecylprodigiosin were cloned as the TFDs, and each TFD was introduced into _S. lividans_ individually. Multiple copies of the TFDs that exist in the cell can bind to repressors, leading to a reduction of the binding between repressors and promoters, leading to activation of the silent genes. They found that two TFDs, RsliM14I and RsliM14 II, can activate actinorhodin, and two TFDs, RsliM18I and RsliM18 III, can activate undecylprodigiosin. Further, the TFD strategy with a low-throughput screen was implicated in _S. coelicolor_ A3(2), _S. albus_ J1074, and _Streptomyces_ sp. F-5635. This strategy was employed to activate 11 silent BGCs, whereas four silent BGCs (36% success rate) were successfully activated, in which butyloctal A (Figure 2) was activated in _Streptomyces_ sp. F-5635.  
In the case of the target BGCs that synthesize color compounds, screening is easy. However, for the colorless compounds that are not easily identified (the compounds absorb in the UV region, or do not absorb the light), it increases the challenge for deducing the results of activation. To find a solution, the TFD strategy was developed by combining with reporter-guided strategy and using the high-throughput screening approach. The TFDs were constructed with a reporter gene (example: GFP) to make a library of vectors. This library of vectors was transformed into native _Streptomyces_ host, and screening based on the expression of the reporter gene can assist in the selection of the recombinant strains (Figure 2). This strategy was used for activating 10 silent BGCs in _Streptomyces griseofuscus_ B-5429 and _Streptomyces_ sp. F-4335 responsible for biosynthesis of oxazolepoxidomycin A (Figure 2), whereas five of them (50% success rate) were successfully activated. This strategy is simpler than other strategies that use deletion or genome editing, such as the deletion of a negative regulator. However, the efficiency of this approach is not high at about 36-50% for the activation of silent BGCs [63]. Therefore, there is still room to improve this strategy by further engineering strategies.