## 1 Actinobacteria and Their Primary Metabolism as Basis for Secondary Metabolism Precursor Supply
### Primary Metabolism in Actinobacteria: Nitrogen Metabolism  
#### 1.2.1 Biochemistry of Nitrogen Metabolism  
Actinobacteria use a variety of nitrogen sources for growth, including simple compounds like ammonium, nitrate and nitrite [6] as well as complex organic sources of nitrogen like amino acids (e.g., glutamate, glutamine, histidine and arginine) [27; 28], urea, amino sugars, peptones [29] and other nitrogen-containing compounds like monoamines and polyamines [8]. Furthermore, Actinobacteria from the genus _Frankia_ are able to utilize atmospheric nitrogen for the production of organic nitrogen compounds [30]. Generally, ammonium is the preferred nitrogen source. It represses the use of other nitrogen sources, e.g., nitrate [31]. Under high concentrations, ammonium can also inhibit the growth of  
Figure 1: A scheme of the main regulatory interconnections of carbon, nitrogen and phosphate metabolisms in _Streptomyces oceliolor_. Arrows indicate positive effects, while dashed arrows indicate negative effects. The dotted lines indicate an uncharacterized or indirect regulation (modified from [25; 26]).  
most Actinobacteria [32]. The diverse nitrogen sources are converted to glutamate and/or glutamine. These amino acids serve as amino donors, mostly in transamination reactions to form other nitrogen-containing compounds.  
Most Actinobacteria possess a functional glutamine synthetase, GlnA (also referred as GlnA1), that catalyzes glutamine synthesis from ammonium and glutamate under low nitrogen concentration conditions. Species of some genera, e.g., _Streptomyces_, have two glutamine synthetases, encoded by the genes _GlnA_ and _GlnII_. Both glutamine synthetases GlnA and GlnII convert glutamate to glutamine, while the major glutamine synthetase activity comes from GlnA [33]. GlnII was first identified in _Rhizobium_. It was hypothesized that GSII may have been obtained by gamma-Proteobacteria from plants through horizontal gene transfer at an early stage of evolution. But, further studies demonstrated that GSII originated from bacteria [34]. Plants seemed to have acquired GSII via chloroplasts. GSII has been found in many representatives of the Actinomycetes, but it is absent in _Amycolatopsis_, _Mycobacterium_ and _Coryobacterium_[34; 35; 36; 37].  
Interestingly, glutamine synthetase-like enzymes (GlnA2, GlnA3 and GlnA4) have been reported in _Streptomyces_[38; 39; 37] and _Mycobacteria_[40], but also in other antibacterial genera, like _Frankia_, _Microbispora_, _Rhodococcus_ and _Brevbacterium_. These enzymes do not show GS activity; instead, GlnA2 and GlnA3 are gamma-glutamylpolyamine synthetases [38] and GlnA4 is a gamma-glutamylethanolamide synthetase [39].  
While GS is preferably active at low nitrogen concentrations, ammonium is metabolized by glutamate dehydrogenase (GDH) under high ammonium concentration conditions. Some Actinomycetes contain two GDH enzymes that catalyze the conversion of ammonium to glutamate (reductive amination) and the reverse reaction. The glutamate cleavage (oxidative deamination) during ammonium dissimilation is carried out by a catabolic, NADH-dependent glutamate dehydrogenase [41]. GSI and assimilatory GDH do not function side by side in all bacteria. In some other species, e.g., in _S. Ingroscopicus_ and _S. clavuligerus_, alanine dehydrogenase (ADH) is active instead of GDH in ammonium assimilation. ADH catalyzes an amination of pyruvate that leads to alanine formation [41; 42] (Figure 2).  
Figure 2: Schematic representation of primary nitrogen metabolism in _Streptomyces coelicolor_. Black lines represent transport through the cell membrane as well as enzymatic reactions in the cell; dashed line—ammonium diffusion through the cell membrane; grey lines represent the cell membrane. GS, glutamine synthetase; GDH, glutamate dehydrogenase; GOGAT, glutamine-2-oxoglutarate-aminotransferase (modified from [43]).