International Journal of Aquaculture, 2025, Vol.15, No.4, 165-174 http://www.aquapublisher.com/index.php/ija 168 (DabA-dabD) gene cluster was found in some marine diatom genomes, indicating that non-cyanobacteria can also perform complex toxin biosynthesis. The common characteristics of these gene clusters are: the gene arrangement is compact, mostly in tandem structures, usually spanning tens of kb or even hundreds of kb intervals, the core encodes a complex biosynthetic enzyme, and there are often modified enzymes or regulatory elements on the side. This structural characteristic ensures efficient coordination of the toxin biosynthesis process and provides a basis for engineering replication and functional research. 3.3 Gene horizontal transfer and diffusion of toxin synthesis ability Studies have shown that toxin synthesis gene clusters have strong mobility and mostly transfer horizontally between different species or strains. Genomic analysis found that transposase or integrase genes associated with multiple toxin-producing gene clusters existed next to each other. For example, mcy, NDA (cyclic cytoxin) and SXT (numbing) gene clusters can all detect relevant sequences on certain plasmids or transposal elements, suggesting that these large gene clusters may mediate transmission in populations through transposons (Popin et al., 2021). This horizontal transfer phenomenon allows relative populations that do not have the ability to produce toxins to change into toxic types by obtaining gene clusters, causing the rapid spread of toxin production capacity. In recent years, there has also been evidence that genomic rearrangements and gene deletions/repetitions of gene clusters are one of the sources of strain diversity. These studies show that toxin gene clusters are not rigid and unchanged, but are constantly spread and optimized through horizontal gene transfer and structural remodeling during evolution, so that the toxin synthesis capacity can be dynamically distributed in algae populations (Chen et al., 2024). 4 Molecular Regulation Mechanisms of Toxin Production 4.1 Transcriptional regulatory factors and signal transduction pathways The expression of toxin biosynthetic gene clusters is regulated by a variety of transcription factors and signaling pathways. Taking the cyanobacterium microcystis toxin as an example, it is known that the nitrogen fixation regulator NtcA can directly bind to the promoter region of the mcy gene cluster to couple toxin synthesis with nitrogen metabolism. In addition, global regulators Fur (hepcidin) and Sigma factors may also regulate mcy cluster transcription by sensing metal or light changes. In dinoflagellate, although the specific transcription factor recognition mechanism is not clear, studies suggest that signals such as cell cycle, light intensity and nitrogen and phosphorus nutrient status can affect the expression of toxin genes. In terms of signaling, phosphorylation cascades, second messengers (such as circular AMPs), etc. may be involved in the interaction of nuclear factors and toxin genes (Zhu et al., 2016). Overall, the transcriptional regulatory network is complex and multi-level, involving environmental signal perception and fine regulation of transduction into target gene promoters, but the detailed mechanism is still being explored. 4.2 Effects of epigenetic modification on toxin synthesis In addition to direct regulation of transcription factors, epigenetic mechanisms (such as DNA methylation, histone modification) may also participate in the regulation of toxin synthesis. In recent years, some scholars have explored the methylation pattern of the Microcystis genome through single-molecule real-time (SMRT) sequencing and other methods, and found that some regulatory genes have different methylation levels in the poison-producing strains, suggesting that they may affect gene expression activity (Stern et al., 2024). However, there are currently few reports on epigenetic regulation of algae toxin synthesis gene clusters. In the future, chromatin immunoprecipitation sequencing (ChIP-seq) and genome-wide methylomics can be used to evaluate the regulatory effect of methylation, acetylation and other modifications on mcy, sxt, and Ana cluster promoters to reveal the role of epigenetics in the regulation of toxin production (Popin et al., 2021). 4.3 Integration analysis of metabolic networks related to toxin synthesis Toxin synthesis requires a large amount of precursors and energy, so its yield is often associated with the overall metabolic state of algae. Multiomics data integration analysis has been used to study the interconnection of toxin synthesis and other metabolic pathways. Through joint metabolomic-tratome analysis, it was found that conditions
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