The Drosophila tyramine-beta-hydroxylase gene encodes multiple isoforms with different functions

The Tyramine-beta−hydroxylase (Tbh) is required for octopamine synthesis. To better understand the function of Tbh in neurotransmitter synthesis, we analyzed the molecular genetic organization of the Drosophila melanogaster Tbh gene and found that the Tbh gene encodes multiple transcripts. The transcripts differ in their 5’UTR, which results in proteins that differ in their size and putative phosphorylation sites, suggesting that the Tbh function is regulated at translational and posttranslational levels. We generated a new Tbh mutant – TbhDel3 - using FLP/FRT recombination mutagenesis to remove the translational start site still that is present in TbhnM18 mutants. The TbhDel3 mutants share ethanol tolerance and larval locomotion defects with the TbhnM18 mutants. But, they differ in terms of their cellular stress response. To develop normal levels of ethanol tolerance, Tbh is required in a subset of Tbh expressing neurons in the adult brain, which was identified using a newly generated Tbh -Gal4 driver. Taking advantage of a newly generated Tbh antibody serum, we show that one Tbh isoform is expressed in a group of peptidergic Hugin-positive and noradrenergic neurons uncoupling Tbh function from octopamine synthesis. The existence of different functional Tbh isoforms impacts our understanding of the regulatory mechanisms of neurotransmitter synthesis and the function of the octopaminergic neurotransmitter system in cellular processes and the regulation of behavior. Author Summary Vertebrates and insects have structurally identical signaling molecules in their nervous system, such as the neurotransmitter dopamine. But, there are also neurotransmitters that are thought to only occur in the vertebrate or insect brain. Noradrenaline is one such neurotransmitter that regulates flight and fight responses in vertebrates. In insects such as the fruit fly Drosophila melanogaster , the structurally very similar neurotransmitter octopamine is considered to be an invertebrate-specific neurotransmitter that performs similar functions to noradrenaline. The functional similarities also extend to enzymes required for synthesis. Our analysis shows that the enzyme for octopamine synthesis exists in several variations and that the connection between the enzymes and the synthesized neurotransmitter may not be as simple as presumed. Exploiting molecular, behavioral and neuroanatomical studies, we show that different variants might be used in response to different environmental conditions and/or the synthesis of alternative, structurally similar neurotransmitters, such as noradrenaline. These results challenge our view on the functions of octopamine and noradrenaline in the regulation of behavior. ### Competing Interest Statement The authors have declared no competing interest.