# Effect of PAS-LuxR Family Regulators on the Secondary Metabolism of _Streptomyces_
## 3 Regulation of Related Product Biosynthesis by PAS-LuxR Family Regulators
### Regulation of Polyene Macrolide Biosynthesis by PAS-LuxR Family Regulators
#### 3.1.2 Network Regulatory System Formed with LAL and PAS-LuxR Family Regulators  
In comparison with the vertical regulatory system, the PAS-LuxR family regulator may form a network regulatory system with the LAL family regulators in polyene macrolides pathway, such as tetramycin, nystatin, amphoteric, candididin, and NPP. Tetramycin consists of A and B components and is a 26-membered tetraene macrolide produced by _S. alhygroscopicus_ and _S. hygrospinosus_, respectively [32; 33]. The tetramycin pathway in _S. alhygroscopicus_ contains three LAL family regulators (TtmRI, TtmRII, and TtmRIII) and a PAS-LuxR family regulator (TtmRIV). The glycosylation genes (_ttmK_ and _ttmC_) and PKS genes (_ttmS0_, _ttmS1_, _ttmS2_, _ttmS3_, and _ttmS4_) are fully regulated by TtmRIV. The sterol oxidase gene (_ttmE_), oxidoreductase genes (_ttmG_ and _ttmF_), glycosylation gene (_ttmI_), transporter genes (_ttmA_ and _ttmB_), and precursor synthesis gene (_ttmP_) are partially regulated by TtmRIV. The _ttmL_, _ttmD_, _ttmRIV_, _ttmRIII_, _ttmRII_ and _ttmRI_ genes are not regulated by TtmRIV. TtmRIV affects the biosynthesis of tetramycin by regulating the transcription of the 14 genes above. TtmRI can directly regulate the transcription of _ttmK_, _ttmC_, _ttmF_, _ttmF_, _ttmS0_, _ttmS1_, _ttmRIV_, _ttmRIII_, _ttmRII_, _ttmRI_, _ttmRI_, _ttmA_, _ttmB_, and _ttmP_ and indirectly regulate the transcription of _ttmE_, _ttmS2_, _ttmS3_, and _ttmS4_. TtmRII directly regulates the transcription of _ttmG_, _ttmF_, _ttmS0_, _ttmRIV_, _ttmRIII_, _ttmRII_, and _ttmRI_ and indirectly regulate the transcription of _ttmE_, _ttmK_, _ttmC_, _ttmS1_, _ttmS2_, _ttmS3_, _ttmS4_, _ttmI_, _ttmA_, _ttmB_, and _ttmP_. TtmRIII directly regulates the transcription of _ttmE_, _ttmG_, _ttmF_, _ttmS0_, _ttmS1_, _ttmRIV_, _ttmRIII_, and _ttmRI_ and indirectly regulate the transcription of _ttmK_, _ttmC_, _ttmS2_, _ttmS3_, _ttmS4_, _ttmJ_, and _ttmP_. Hence, a network regulatory system formed with TtmRI, TtmRII, TtmRIII, and TtmRIV regulates the biosynthesis of tetramycin [34]. The pathway of the 38-membered tetraene macrolide nystatin in _S. nourse_ has four regulators, namely, LAL family regulators NysRI, NysRII, and NysRIII, and PAS-LuxR family regulator NysRIV. The transcription of the _nysR_ gene, which is responsible for transport, is not regulated by NysRI, but rather by NysRII, NysRIII, and NysRIV [35]. The transcription of glycosylation genes _nysDI_ and _nysDI_ is not regulated by NysR. The transcription of PKS genes _nysA_ and _nysI_ genes is regulated by NysR. The transcription of _nysRI_ and _nysRIV_ is regulated by all the four regulators, although the degree of regulation differs. The transcription of _nysRI_ is moderately regulated by NysRIII, NysRIV, and NysRI and fully regulated by NysRII. The transcription of _nysRIV_ is moderately regulated by NysRIV and fully regulated by NysRI, NysRII, and NysRIII [35]. The regulation of NysR to the structural genes and the regulatory genes indicated that they form a network regulatory system. The LAL and PAS-LuxR family regulators are also identified in the candididin, NppB1, and salmonycin pathway. Candicidin is an heptane macrolide that is produced by _S._ sp. FR-008 [36]. The candididin biosynthetic gene cluster involves 21 genes, and four regulatory genes encode the PAS-LuxR family regulator FscRI and the LAL family regulators FscRII, FscRII, and FscRIV. In _AfscRI_, the RT-PCR results show that the transcriptions of the six PKS genes (_fscA_, _fscB_, _fscC_, _fscD_, _fscE_, and _fscF_), the glycosylation genes (_fscMI_, _fscMII_, and _fscMIII_), and the translocation genes (_fscT1_ and _fscTII_) were reduced compared with the wild-type strain, suggesting that FscRI regulates the biosynthesis of candididin by affecting the transcription of all the above genes. FscRII and FscRIII belong to the top of the regulatory network in which FscRII negatively regulates the expression of FscRIII and positively regulates the expression of FscRIV. FscRIII has a positive regulatory effect on FscRI and FscRII.