Identification of the RNAs for Transcription Factor Mitf as a Component of the Balbiani Body

Balbiani body (BB) is a large distinctive organelle aggregate uniquely present in developing oocytes of diverse animal species. BB is thought as a stage-specific structure that resembles germ plasm, the cytoplasmic organelle of germ cells. The role and function of BB have remained speculative because of a highly dynamic structure and a lack of genetic and molecular data. BB has been found to contain proteins and RNAs, none of them – except the zebrafish foxH1 RNA, is or encodes a transcription factor. Here we report in the fish medaka (Oryzias latipes) that RNAs encoding microphthalmia-associated transcription factor (Mitf) are prominent components of the BB. By fluorescence in situ hybridization on ovarian section, we revealed that the transcripts of both mitf1 and mitf2 genes concentrated in the BB, in which they co-localized with the dazl RNA, a definitive BB marker highly conserved in vertebrates. Therefore, the mitf product may play dual roles in germ gene transcription and BB formation and/or function in this organism. Our data provide the second evidence that the RNA of a transcription factor can be a prominent component of the BB in a vertebrate.