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Abstract MYC transcription factors have critical roles in facilitating a variety of cellular functions that have been highly conserved among species during evolution. However, despite circumstantial evidence for an involvement of MYC in animal osmoregulation, mechanistic links between MYC function and osmoregulation are missing. Mozambique tilapia (Oreochromis mossambicus) represents an excellent model system to study these links because it is highly euryhaline and highly tolerant to osmotic (salinity) stress at both the whole organism and cellular levels of biological organization. Here, we utilize anO. mossambicusbrain cell line and an optimized vector-based CRISPR/Cas9 system to functionally disrupt MYC in the tilapia genome and to establish causal links between MYC and cell functions, including cellular osmoregulation. A cell isolation and dilution strategy yielded polyclonalmyca(a gene encoding MYC) knockout (ko) cell pools with low genetic variability and high gene editing efficiencies (as high as 98.2%). Subsequent isolation and dilution of cells from these pools produced amycako cell line harboring a 1-bp deletion that caused a frameshift mutation. This frameshift functionally inactivated the transcriptional regulatory and DNA-binding domains predicted by bioinformatics and structural analyses. Both the polyclonal and monoclonalmycako cell lines were viable, propagated well in standard medium, and differed from wild-type cells in morphology. As such, they represent a new tool for causally linkingmycato cellular osmoregulation and other cell functions. 

Quantitative DIA proteomics in combination with the transcription inhibitor actinomycin D was performed on the tilapia OmB cell line to identify proteins that are upregulated by transcriptional regulation during hyperosmotic stress. This analysis revealed proteins that are transcriptionally up-regulated by hyperosmolality in these cells. The promoter regions of these proteins were compared and a novel hyperosmolality-induced cis-regulatory element (CRE) and corresponding transcription factor candidate were identified. The CRE was experimentlly validated by site-directed mutagenesis in combination with reporter assays.