Abstract
<title>Abstract</title> <p> <italic>Aetokthonos hydrillicola</italic> is a cyanobacterium that produces the potent neurotoxin aetokthonotoxin (AETX), a pentabrominated indole alkaloid accumulating in the food chain, and the causative agent of Vacuolar Myelinopathy (VM). Because AETX biosynthesis occurs only when bromide is available, we investigated how bromide supply influences transcription of the <italic>aet</italic> biosynthetic gene cluster ( <italic>aet</italic> BGC), examining both short-term (minutes to hours) and long-term (days) transcriptional responses. While AETX production was strictly dependent on bromide availability, with higher bromide doses resulting in higher toxin concentrations, the <italic>aet</italic> transcription did not always coincide with the pace outlined by toxin production. Although bromide caused a fast dose-dependent upregulation of our <italic>aet</italic> reporter gene ( <italic>aetF</italic> ) within hours, on the scale of days <italic>aetF</italic> transcription was decoupled from toxigenesis, suggesting a broader physiological regulation centered around an emerging nitrogen limitation. Bioinformatic analysis of the <italic>aet</italic> BGC identified putative binding sites for NtcA, a global transcriptional regulator known to govern the nitrogen metabolism in cyanobacteria, as well as binding sites matching for the T-N11-A box, characteristic of LysR-type transcriptional regulators (LTTRs), indicating a new example of interplay between NtcA and cyanobacterial secondary metabolism. These findings illustrate the intertwined regulation of the primary and secondary metabolisms in cyanobacteria, and emphasize that full understanding of the environmental drivers of VM requires further investigation of how factors beyond bromide availability, such as ambient dissolved nitrogen levels, influence <italic>aet</italic> expression. </p>