A cis‐regulatory Element in Intron 1 Cooperates with the Core Promoter during Salinity Induction of Tilapia Glutamine Synthetase

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Mozambique tilapia (O. mossambicus) is a euryhaline fish that tolerates salinity ranging from fresh water (FW) to 120 g/kg (> 3x seawater, SW). Hyperosmotic stress associated with salinity transfer from FW to SW strongly induces glutamine synthetase (GS) transcription in tilapia, including a brain cell line model (OmB cells). In this study we cloned the 5′ regulatory sequence (RS) (including 5′‐flanking region and 5′‐UTR) of the O. mossambicus GS gene to identify osmotic response elements that mediate transcriptional induction. Using enhancer trapping, we discovered a novel osmosensitive mechanism that utilizes an unusual enhancer cassette in intron 1 of the GS gene. This cassette includes a single copy of a cis‐regulatory element (CRE), the osmolality/salinity‐responsive element 1 (OSRE1). We show that intronic OSRE1 is necessary but not sufficient for intron 1‐mediated hyperosmotic GS induction. Furthermore, intron 1 and its interaction with the GS core promoter complex are also necessary for hyperosmotic GS induction. We propose a novel model for hyperosmotic intron 1‐mediated induction of the O. mossambicus GS gene that is based on the interaction of an OSRE1 enhancer, intron 1, and the core GS promotor.Support or Funding InformationThis study was funded by a grant from NSF (IOS‐1656371).