## 3 Semi-Synthetic and Molecular Activation of Silent BGCs
### Artificial Promoters  
While attention has focused on the manipulation of transcriptional activation of BGCs, another approach has been the insertion of inducible artificial promoters to drive expression of the silent genes. This generally takes advantage of the advances in molecular cloning technologies where suitable plasmid systems are generated in which structural biosynthetic genes can be cloned downstream of strong promoters, thus overcoming the cryptic native transcription network. Decoupling pathway expression from the complexity of native regulation circumvents the need for culture specific conditions and in some cases the laborious search for appropriate activating signals. In addition, production of the respective natural products can be controlled under defined culture conditions, ensuring downstream processing and stabilization prior to isolation. This has received a lot of attention in fungal systems where the transcriptional control of large clusters can be manifestly complex [18]. Described as refactoring, already this approach has met with some success, as with activation of the silent spectinabilin and taroomycin A pathways from _S. orinoci_ and _Saccharomyces_ sp. CNQ-490, respectively [4; 58]. In the latter case, transformation-associated recombination (TAR) cloning was exploited to express a 67-kb nonribosomal peptide synthetase BGC from the marine actinomycete _Saccharomyces_ sp., producing the dichloromethane lipopeptide antibiotic taroomycin A in the model expression host _S. coelicolor_[4]. In another study, [PERSON] and colleagues used a similar strategy to activate a cryptic BGC SGR810-815 from _S. griseus_, resulting in the production of three novel polycyclic tetramate macrolactams [106]. In what the authors of each study described as a \"plug and play scaffold\", the respective host strains were engineered using a specific set of heterologous promoters that were functional in a heterologous host under the target culturing condition. More recently, a single genomic capture and expression vector for antibiotic production in _Bacillus subtilis_ has been reported [107], while the pathway for production of alterchromide lipopeptides by _Pseudoalteromonas piscicida_ JCM 20779 was heterologously expressed in _E. coli_ utilizing native and _E. coli_-based T7 promoter sequences [108]. Heterologous expression of genes from the proposed aspyridone biosynthetic cluster from _A. nidulans_ in the host _A. oryzae_ led to the production of eight different compounds in addition to aspyridone A 1, one of the previously observed products [18]. Surprisingly, the previously accepted final product of the pathway, aspyridone B 2, was not detected [18]. Similarly, expression of silent gene clusters has also been achieved in _Streptomyces_ using the strongly constitutive promoter emE*p [109], while introduction of the transcriptional activator of balhimycin biosynthesis, the _bbr_ gene from _Amycolatoppsis balhimycina_ (_bbrAba_) into _A. japonicum_ also resulted in the production of an antibiotic-active compound [110]. More recently, a new high-performance heterologous fungal expression system was described in _Aspergillus terreus_[111]. Based on regulatory elements from the terrein gene cluster, the system was found to be particularly suitable for high level expression of polyketides in heterologous hosts. However, despite some successes, challenges remain and molecular-based methods utilizing heterologous expression systems continue to be limited by problems such as locating and cloning genes, difficulties with gene transformation and inactivation, and host incompatibilities [32].