## 3 Utilization of Precursor Compounds from Primary Metabolism in Secondary Metabolism  
### Secondary Metabolism in Actinobacteria  
In soil, nutrients are usually present in diverse complexity and abundance. Microorganisms need tools to sense, modify and use these compounds for biomass and secondary metabolite production. The soil is a complex and dynamic environment, and the activity of microorganisms plays a crucial role in multiple geochemical cycles. Secondary metabolites are not essential for the growth of the producer organisms, but they are important in various survival strategies [26; 81]. Actinomycetota, which includes _Streptomyces_, is one of the largest bacterial groups participating in metabolic recycling in soil and the production of many antibiotics and other secondary metabolites such as antifungals, antivirals, antibacterials, immunodifiers, enzyme inhibitors, antitumor drugs, insecticides, herbicides, etc. In _Streptomyces_, secondary metabolites are compounds that protect cells from environmental stresses and selective pressures [82]. For example, siderophores improve the absorption of iron from the environment; their biosynthesis is induced by iron deficiency in the cell, which can have a growth-promoting effect on the host organism [83]. Other examples are carotenoids, colored terpenoids that protect against photo-oxidative damage and oxygen radicals [84]. Other examples are pigments like melanin that can protect against UV damage [85]. Furthermore, another group of secondary metabolites is terpenoids, which can act as antibiotics, odorants, hormones and flavorings, e.g., a tricyclic sesquiterpene antibiotic albaflavenone [86] and ectoines that are effective against osmotic stress and are able to prevent protein misfolding [87]. Also, hopanoids were shown to be formed during the transition from substrate to aerial hyphae [88]. Other groups of compounds belonging to the group of antibiotics that can inhibit bacterial growth also represent a fitness advantage in the fight for nutrients. Secondary metabolites produced by _Streptomyces_ can allow for the interaction with higher organisms. One of these well-studied symboises is between the streptomycin cote _Streptomyces hygroscopic_ and the fungus _Aspergillus nidulans_, in which, secondary metabolite production is induced after direct physical contact between the two partners. The produced becanor acid inhibits ATP biosynthesis and induces a symbiosis between the fungus and streptomycin cote [89]. Actinomycetes-derived antibiotics important for medicine include tetracyclines, aminoglycos, macroclides, anthracyclines, glycopeptides, etc. [53; 90]. The production of antibiotics is a very complex process involving different special genes, which are mostly located in gene clusters. In addition to the genes for the actual biosynthesis, genes for the transport, regulation and mediation of resistance are often included [91]. Due to the chemical structure and different modes of action, antibiotics can be can be divided into different classes according to biological activity: anti-infective antibiotics (antibacterial, antifungal and antiviral), cytostatics, herbicides, insecticides, acaricides and nematozoids. They can also be divided according to the chemical structure: lactams, aminoglycos, macrolides, tetracyclines, limcosamides, sulfonamides, nitroimidazoles, polyketide antibiotics and polypeptide antibiotics. _Streptomyces_ spp. are a large source of biologically active substances [2]. These include a variety of active compounds that are applied in numerous fields in medicine and agriculture. _Streptomyces_ spp. produce antibacterials, antifungals, immunosuppressants and other bioactive compounds [92; 93]. The secondary metabolites produced by Actinobacteria show a great variety, and their underlying chemical structure classes are also diverse, e.g., polyketides, polypeptides, lipoproteins, glycosides, terpenoids and alkaloids. Despite the great diversity, many natural products are based on similar biosynthetic principles.