Alternative splicing of medaka bcl6aa and its repression by Prdm1a and Prdm1b

Abstract Bcl6 and Prdm1 (Blimp1) are a pair of transcriptional factors that repressing each other in the mammals. Prdm1 represses the expression of bcl6 by binding a cis-element of bcl6 gene in mammals. The homologs of Bcl6 and Prdm1 have been identified in teleost fish. However, whether these two factors regulate each other by the same way in fish as that in the mammals is not clear. In this study, the regulation of bcl6aa by Prdm1 was investigated in medaka. The mRNA of bcl6aa has three variants (bcl6aaX1-X3) at the 5′-end by alternative splicing with different promoters detected by RT-PCR. The three variants can be detected in adult tissues and developing embryos of medaka. The predicted proteins of Bcl6aaX1-X3 may have modification such as acetylation, C-mannosylation, phosphorylation, and sumoylation in the N-terminuses with different half-lives and relative translation efficiencies. Prdm1a and prdm1b are expressed in the tissues and embryos where and when bcl6aa is expressed. The expression of prdm1a was high while the expression of bcl6aa was low, and vice versa, detected in the spleen after stimulation with LPS or polyI:C. In vitro reporter assay indicated that bcl6aa could be directly repressed by both Prdm1a and Prdm1b in a dosage-dependent manner. After mutation of the key base, G, of all predicted binding sites in the core promoter region of bcl6aa, the repression by Prdm1a and/or Prdm1b disappeared. The consensus binding site of Prdm1 in bcl6aa gene is GAAAA(T/G). These results indicate that both Prdm1a and Prdm1b directly repress the expression of bcl6aa by binding the consensus binding site where the 5′-G is critical in medaka fish.