# Bacterial Semichemicals and Transkingdom Interactions with Insets and Plants
## 3 BVOC Biosynthetic Pathways and Their Intersections with Primary and Secondary Metabolism
### BVOCs and Secondary Metabolism
#### 3.2.1 Pyrazine Metabolism in Bacteria  
Pyrazines, an important class of BVOCs with chemical signaling properties in insects, are secondary metabolites that have been known since the early 1970s for their use in agriculture (insecticides and nematicides) or medicine (antidepressants, diuretics, and antibacterial) [110]. More recently, these compounds were described as flavor ingredients in the industries of chocolate and coffee [111], and also as anti-neoplastic [112] and anti-HIV drugs [113]. The pyrazine biosynthetic pathways exemplify how primary and secondary metabolisms are strictly interconnected. In general, in a secondary metabolism biosynthetic pathway, the first enzyme of a secondary biosynthetic pathway controls the metabolic flux, channeling precursors from the primary metabolites (amino acids, in pyrazine biosynthesis) toward specialized secondary metabolites. Thereafter, different sets of reactions occur for precursors assembling in the molecular skeleton and post-assembly modification. Production of pyrazines for commercial purposes have allowed a better understanding of both the physiology of the pyrazine-producing microorganisms and the details of the biosynthetic pathways underlying their synthesis. Although there are many studies that have treated the biosynthesis and production of bacterial pyrazines as secondary metabolites, only few studies have reported their possible activity as semiochemicals in insects. We know, for instance, that 2,5-dimethylpyrazine is an important pheromone in ants _Atta sexdens_ and it is produced by _Citrobacter freundii_[114] and _Microbacterium fildiorum_[115]. In addition, 2,5-dimethyl-pyrazine and other pyrazines (2,3,5-trimethyl-pyrazine and tetramethyl-pyrazine) are produced by bacteria of the genus _Bacillus_ by industrial fermentations [116]). [PERSON] and coworkers [117] reviewed some industrial applications of microbial pyrazines, the producer strains, and the proposed biosynthetic pathways. Many studies demonstrated that pyrazine biosynthesis is tightly linked to amino acid metabolism. For examples, 2,5-disopropyl-pyrazine, 2-isopropyl-5-secbutyl-pyrazineand, and 17 other pyrazines were produced upon valine feeding of _Paenibacillus polymyxa_ cultures [118]. These BVOCs are also produced by bacteria of the genus _Exiguobacterium_ and act as oviposition signals in _Anopheles gambiae_[32,33]. In yeasts of the genus _Aspergillus_ (_Aspergillus flavus_ and _Aspergillus oryzae_), a similar experiment of amino acid feeding demonstrated that leucine and isoleucine supplementation of the culture medium enhanced the production of aspergillic acid [119]. This compound has antibacterial and chemoprotective activities [120,121]. Other pyrazines produced by the yeast of the genus _Aspergillus_ (_Aspergillus scleroortium_[122] and _Aspergillus parasiticus_[123]) are listed in Table S1. The proposed biosynthetic mechanism is based on condensation reaction between leucine and isoleucine and is depicted in Figure 4.  
Figure 4: Pyrazines biosynthesis. Generally, experimental data show a correlation between amino acids metabolisms and pyrazine production in microorganisms. Yeast species, if cultured in isoleucine or leucine-rich medium, produce pyrazines used in an industrial process (**A**) _Aspergillus_ produces aspergillic acid and similar molecules, _Candida pulcherrima_ produces pulcherriminc acid. A similar effect (**B**) is observed in _Pseudomonas perolens_, adding valine and glycine in the culture broth. In this last case, there was a proposed pathway. Finally, adding radiolabeled precursors, a pathway was individuated in _Serratia marcscens_ B2 that start from threonine (**C**). Another pyrazine, pulcherriminic acid, (2,5-diisobutyl-3,6-dihydroxypyrazine-1/4-dioxide) is synthesized by various bacteria, including _Bacillus subtilis_, _Microoccus violagabriellae_, and _Paenibacillus maceran_, as well as by the yeast _Candida pulcherrima_[119; 120; 121; 122; 123; 124; 125; 126; 127]. In _B. subtilis_ and _C.