# Naturally Occurring Flavonoids and Isoflavonoids and Their Microbial Transformation: A Review
## 4 Development and Utilization of Flavonoids Synthesis Genes in _Streptomyces_  
The availability of flavonoids in plants is limited by seasonal or regional variations, low abundance, and instability in the separation of single compounds from complex mixtures. Therefore, expressing plant biosynthesis genes in microorganisms is an effective method to produce these essential metabolites [97]. _Streptomyces_, one of the most important genera of actinomycetes, is diverse, and more than 50% of its species can produce various antibiotics and other active compounds of different structural types [98; 99]. Enzymes with various catalytic functions in _Streptomyces_ are useful members of an artificial gene cluster constructed in _E. coli_ for the production of plant-specific flavones, including isoflav and unnatural compounds by fermentation [100; 101]. Here, we summarize the genes involved in flavonoid synthesis. The most common method of studying genes that produce flavonoids in _Streptomyces_ is the introduction of gene fragments into _E. coli_ for heterologous expression. In plants, chalcone is a precursor of flavonoid, which can be converted to flavanones by chalcone isomerase, and 4-coumarate: coenzyme A ligase (4 CL) is an essential enzyme in the transformation process. The gene fragment that synthesises 4 CL has been found in _S. coelicolor_ A3(2) and can be introduced into _E. coli_ for heterogenic expression, which greatly enhances flavonoid production [100; 102]. In a study by [PERSON] et al., the SaOMT-2 gene from _S. avermitilis_ MA-4680 was inserted into _E. coli_ Sa-2 for expression, and the enzyme encoded by the SaOMT-2 gene was found to methylate the 7-hydroxyl group of isoflav and flavonoids. In particular, the biotransformation of _E. coli_ Sa-2 resulted in the production of sakuranetin, an antifungal flavonoid [103]. A gene fragment from _S. peucetius_ ATCC 27952 that encodes an O-methyltransferasease (OMT), SpOMT2884, was expressed in _E. coli_, and the enzyme was found to methylate various flavonoids, with 7,8-dihydroxyflavone being the most favourable substrate. Bioconversion of 7, 8-dihydroxyflavone was as high as 96% under optimal conditions, and the resulting 7-hydroxy-8-methoxyflavone was purified for in vitro glycosylation to produce glucose. The results indicated that methylation enhances the stability of the substrate, whereas glycation was shown to increase the water solubility of the substrate [104]. The SpOMT7740 gene from _S. peucetius_ ATCC27952 encoding an OMT was cloned into _E. coli_ to methylate different types of flavonoid substrates. The enzyme catalyzes the formation of various natural and non-natural \(O\)-methoxides [95]. \(O\)-Methylated phenylpropanoids are usually found in small quantities in plants. However, studies have indicated that co-culture of _E. coli_ and _Streptomyces_ can activate the expression of the SaOMT2 gene and promote the production of \(O\)-methylated phenylpropanoids, thus providing a powerful pathway for the production of scarce and valuable \(O\)-methylated phenylpropanoids [24]. Studies have shown that methylase GerMIII, derived from _Streptomyces_ sp. KCTC 0041 BP, can methylate the 4-hydroxyl position of some flavonoids, and quercetin may be the most suitable substrate [105]. The gene encoding oleandomycin glycosyltransferase (OleD GT) from _S. antibioticus_ was introduced into _E. coli_ BL21(DE3) for expression, and the purified recombinant OleD GT catalyzed glycosylation of various flavonoids. Fixation of OleD GT in hybrid nanoparticles of Fe\({}_{3}\)O\({}_{4}\)/silica/NiO is an effective technique for maintaining its high activity [106]. Not only _E. coli_ but also _Streptomyces_ can be used as engineered strains for heterologous expression of flavonoid-producing genes. In a study, flavonoid and stilbene biosynthesis genes were introduced for heterologous expression in _S.