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SECTION I: MOLECULAR GENETIC ENGINEERING AND BIOCHEMICAL TECHNOLOGY 2535. SYNTHESIS OF NEUROTRANSMITTER PRECURSORSNeurotransmitters such as serotonin, dopamine, and norepinephrine play a crucial role in regulating brain functions, including cognition, attention, sleep, and mood. The regulation of neurotransmitter levels is one of the pathogenic mechanisms of depression, related to the deficiency of these neurotransmitters. This hypothesis opens up avenues for research into the treatment of depression by increasing neurotransmitter concentrations. Compounds that inhibit the reuptake of neurotransmitters at the synaptic cleft have been shown to be clinically effective in treating depression. Similarly, compounds that inhibit monoamine oxidase - a type of enzyme that breaks down neurotransmitters - also increase neurotransmitter concentrations in presynaptic neurons. Additionally, probiotic therapies with the ability to produce neurotransmitters and their precursors have also received considerable attention [21].Serotonin (5-HT) plays a key role in regulating mood and behavior and is also involved in the synthesis pathway of melatonin, a hormone that regulates the biological clock and sleep. Numerous studies have highlighted the link between 5-HT and the pathogenic mechanisms of depression, with the effects of tryptophan deficiency, a precursor of 5-HT, exacerbating depressive symptoms [22]. Several Bacillus species, such as B. licheniformis, B. acidocaldarius, and B. subtilis, have the ability to produce L-tryptophan [7, 8], providing a source of material for 5-HT synthesis. Additionally, some strains can produce other intermediates in the synthesis of serotonin and melatonin, such as B. amyloliquefaciens, or directly produce melatonin, like B. thuringiensis [23].In addition to 5-HT, depression is also associated with a reduction in norepinephrine (NE) and dopamine (DA). Both of