LAUSR

Polyene macrolides are a group of natural products with potent antifungal activity (Caffrey et al., 2016). Candicidin/ FR-008, a potent broad-spectrum anti-fungal agent, is produced by several Streptomycetes, including Streptomycetes sp. strain FR-008 (Chen et al., 2003), S. griseus 3570 (Campelo and Gil, 2002) and S. albus J1074 (Olano et al., 2014). Due to its medical importance, considerable effort has been applied to elucidate its biosynthetic pathway and identify key regulatory genes (Chen et al., 2003; Zhang et al., 2015). The candicidin biosynthetic gene cluster had been cloned, and at least 21 genes for candicidin biosynthesis were proposed, which includes type I polyketide synthases (PKSs), type II thioesterases, post-PKS tailoring enzymes, and four putative pathway-specific regulators (Chen et al., 2003) (Figure S1 in Supporting Information). FscRI, an activator in candicidin biosynthesis, belongs to the LuxR family with a N-terminal PAS sensory domain and a C-terminal HTH motif. FscRI controls the transcription of most genes in the candicidin gene cluster by direct binding to their promoters. FscRIV constitutes a secondary transcriptional activator of candicidin biosynthesis. It belongs to Streptomyces antibiotic regulatory protein (SARP) family of transcriptional activators. Inactivation of fscRIV resulted in significant decrease of candicidin production (Zhang et al., 2015). Manipulation of pathway regulation has been reported as an efficient strategy to increase secondary metabolite production (Yin et al., 2019). To identify global regulators associated with secondary metabolism, several different strategies have been employed. Affinity purification of biotinylated promoter of key structural genes as a probe has yielded several novel regulators, such as TetR family transcriptional regulators AtrA and DepR1 for daptomycin production in S. roseosporus, BldD for erythromycin production in Saccharopolyspora erythraea (Chng et al., 2008; Mao et al., 2015). Forward genetic screen using transposonbased mutagenesis has also identified candidate genes that are involved in tripyrrole antibiotic undecylprodigiosin (RED) and Actinorhodin production in S. coelicolor (Xu et al., 2019). To identify global regulators involved in candicidin biosynthesis, we screen a mini-Tn5 transposon library of S. albus J1074 to find mutants with impaired candicidin production. We identify dozens of novel genes involved candicidin regulation (Table S1 in Supporting Information). They involved in diverse cellular processes such as amino acid metabolism, lipid metabolism, transcriptional regulation and unknown functions (Figure 1A). A mutant with a transposon insertion in the coding region of putative xenobiotic response element (XRE) family regulator XNR_2583 (CanR1) was selected for further analysis in this study (Figure 1B). CanR1 has the typical characteristics of the XRE-family regulators, which contains an XRE-family HTH (helix-turn-helix) DNA-binding domain at its N-terminus. CanR1 and its