B‐cell lymphoma‐6 (Bcl6) is a transcriptional repressor that plays important roles in various physiological activities such as innate and adaptive immune response, lymphocyte differentiation, and cell cycle regulation in mammals. Two homologs of Bcl6a, namely Bcl6aa and Bcl6ab, are identified in teleost fish including medaka Oryzias latipes. The expression profiles of bcl6aa and bcl6ab in medaka were studied using reverse‐transcription polymerase chain reaction and in situ hybridization. The transcripts of bcl6aa and bcl6ab were detected from very early embryos such as the four‐cell stage until hatching. Bcl6aa and bcl6ab were clearly detected in the embryonic body from 5 days postfertilization onward by in situ hybridization. Bcl6aa was specifically expressed in the retina, whereas bcl6ab was expressed in entire embryonic body. The results referred to that both bcl6aa and bcl6ab originate maternally in the zygotes and may play major roles in embryogenesis of medaka. The transcripts of bcl6aa and bcl6ab were detected in all examined adult tissues, including immune organs such as the gill, spleen, kidney, liver, and intestine. The expression of bcl6aa and bcl6ab in the liver, spleen, head–kidney, and intestine could be upregulated or downregulated by lipopolysaccharide and polyriboinosinic–polyribocytidylic acid. These results indicate that both bcl6aa and bcl6ab may be involved in immune response in medaka.

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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 identi ed in teleost sh. However, whether these two factors regulate each other by the same way in sh 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 modi cation such as acetylation, C-mannosylation, phosphorylation, and sumoylation in the N-terminuses with different half-lives and relative translation e ciencies. 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 sh.

Protein arginine methylation is important for gene regulation and biological processes. Methylosome protein 50 (Mep50) is identified as a partner of protein arginine methyltransferase 5 (Prmt5), a major enzyme capable of symmetric dimethylation, in mammals and Xenopus. The isolation and characterization of medaka mep50 were reported in this paper. Medaka Mep50 is a homolog of human MEP50 with six WD40 domains. Medaka mep50 was ubiquitously expressed in the adult tissues and had maternal origin with continuous and dynamical expression during embryonic development detected by RT-PCR and in situ hybridization. A strong interaction of medaka Mep50 and Prmt5 was shown by yeast two hybridization. The expression pattern of mep50 is similar to that of prmt5 in medaka. The results suggested that medaka Mep50 could be a partner of Prmt5 and might play major roles in a variety of tissues in medaka.