Activation and mechanism of a cryptic oviedomycin gene cluster via the disruption of a global regulatory gene, adpA, in Streptomyces ansochromogenes.

Genome sequencing analysis has revealed at least 35 clusters of likely biosynthetic genes for secondary metabolites in Streptomyces ansochromogenes. Disruption of adpA encoding a global regulator (AdpA) resulted in the failure of nikkomycin production, whereas other antibacterial activities against Staphylococcus aureus, Bacillus cereus, and Bacillus subtilis were observed with the fermentation broth of ΔadpA but not with that of the wild-type strain. Transcriptional analysis showed that a cryptic gene cluster (pks7), which shows high identity with an oviedomycin biosynthetic gene cluster (ovm), was activated in ΔadpA. The corresponding product of pks7 was characterized as oviedomycin by MS and NMR spectroscopy. To understand the molecular mechanism of ovm activation, the roles of six regulatory genes situated in the ovm cluster were investigated. Among them, proteins encoded by co-transcribed genes ovmZ and ovmW are positive regulators of ovm AdpA directly represses the transcription of ovmZ and ovmW Co-overexpression of ovmZ and ovmW can relieve the repression of AdpA on ovm transcription and effectively activate oviedomycin biosynthesis. The promoter of ovmOI-ovmH is identified as the direct target of OvmZ and OvmW. This is the first report that AdpA can simultaneously activate nikkomycin biosynthesis but repress oviedomycin biosynthesis in one strain. Our findings provide an effective strategy that is able to activate cryptic secondary metabolite gene clusters by genetic manipulation of global regulatory genes.

Reconstruction of a hybrid nucleoside antibiotic gene cluster based on scarless modification of large DNA fragments.

Genetic modification of large DNA fragments (gene clusters) is of great importance in synthetic biology and combinatorial biosynthesis as it facilitates rational design and modification of natural products to increase their value and productivity. In this study, we developed a method for scarless and precise modification of large gene clusters by using RecET/RED-mediated polymerase chain reaction (PCR) targeting combined with Gibson assembly. In this strategy, the biosynthetic genes for peptidyl moieties (HPHT) in the nikkomycin biosynthetic gene cluster were replaced with those for carbamoylpolyoxamic acid (CPOAA) from the polyoxin biosynthetic gene cluster to generate a ~40 kb hybrid gene cluster in Escherichia coli with a reusable targeting cassette. The reconstructed cluster was introduced into Streptomyces lividans TK23 for heterologous expression and the expected hybrid antibiotic, polynik A, was obtained and verified. This study provides an efficient strategy for gene cluster reconstruction and modification that could be applied in synthetic biology and combinatory biosynthesis to synthesize novel bioactive metabolites or to improve antibiotic production.