Identification of a T-box transcription factor TBX2 and its negative regulatory role in melanin production in the Pacific oyster Crassostrea gigas.

Oyster shells exhibit varying color patterns-black, white, or black and white striations-attributable to differences in melanin content and distribution. In this study, we identified a new homolog of TBX2, a member of the T-box transcription factor family, in the Pacific oyster (Crassostrea gigas) named CgTBX2. The mRNA expression of CgTBX2 was higher in tissues from white-shelled oysters than in those from black-shelled oysters. Knockdown of CgTBX2 in C. gigas significantly upregulated the expression of two key genes involved in melanin synthesis, CgTYR and CgTYR2. The tissue expression profile revealed that CgTBX2 is highly expressed in gill and mantle. CgTBX2 protein is mainly localized in the nucleus and can combine with TBX-binding elements (TBEs) present in the CgOCA2 promoter. CgTBX2 repressed the activity of the CgOCA2 promoter, with a strong transcriptional inhibition effect on the -2000 to -878 and -211 to -1 regions. Furthermore, the combination of CgTBX2 with GTTGACACCTT sequence is responsible for the transcriptional inhibition on the -211 to -1 region of the CgOCA2 promoter. Our findings reveal that CgTBX2, a novel regulator of melanin pigmentation in C. gigas, negatively regulates melanin deposition by inhibiting tyrosinase genes expression and repressing the transcriptional activity of the CgOCA2 promoter.