[Transcriptional regulation of an enzyme involved in the synthesis of melatonin].

HIOMT mRNA levels increased by 100% in constant light compared to constant darkness.

• The enzyme HIOMT is crucial for melatonin synthesis in the chicken pineal gland.
• There is a day/night rhythm in HIOMT gene transcription, with mRNA levels three times higher at midday than at midnight.
• Unexpected darkness does not inhibit the daytime rise in HIOMT mRNA levels.
• Unexpected light prevents the nighttime decrease in HIOMT mRNA levels.
• The findings suggest that HIOMT gene expression is regulated by both light exposure and a biological clock.

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