Expression of the vasa gene is associated with germline establishment. Therefore, identification of vasa activator(s) should provide insights into germline development. However, the genes sufficient for vasa activation remain unknown. Previously, we showed that the BTB/POZ-Zn-finger protein Mamo is necessary for vasa expression in Drosophila. Here, we show that the truncated Mamo lacking the BTB/POZ domain (MamoAF) is a potent vasa activator. Overexpression of MamoAF was sufficient to induce vasa expression in both primordial germ cells and brain. Indeed, Mamo mRNA encoding a truncated Mamo isoform, which is similar to MamoAF, was predominantly expressed in primordial germ cells. The results of our genetic and biochemical studies showed that MamoAF, together with CBP, epigenetically activates vasa expression. Furthermore, MamoAF and the germline transcriptional activator OvoB exhibited synergy in activating vasa transcription. We propose that a Mamo-mediated network of epigenetic and transcriptional regulators activates vasa expression.

Epigenetic silencing is critical for maintaining germline stem cells in Drosophila ovaries. However, it remains unclear how the differentiation factor, Bag-of-marbles (Bam), counteracts transcriptional silencing. We found that the trimethylation of lysine 36 on histone H3 (H3K36me3), a modification that is associated with gene activation, is enhanced in Bam-expressing cells. H3K36me3 levels were reduced in flies deficient in Bam. Inactivation of the Set2 methyltransferase, which confers the H3K36me3 modification, in germline cells markedly reduced H3K36me3 and impaired differentiation. Genetic analyses revealed that Set2 acts downstream of Bam. Furthermore, orb expression, which is required for germ cell differentiation, was activated by Set2, probably through direct H3K36me3 modification of the orb locus. Our data indicate that H3K36me3-mediated epigenetic regulation is activated by bam, and that this modification facilitates germ cell differentiation, probably through transcriptional activation. This work provides a novel link between Bam and epigenetic transcriptional control.

In Drosophila, the expression of germline genes is initiated in primordial germ cells (PGCs) and is known to be associated with germline establishment. However, the transcriptional regulation of germline genes remains elusive. Previously, we found that the BTB/POZ-Zn-finger protein, Mamo, is necessary for the expression of the germline gene, vasa, in PGCs. Moreover, truncated Mamo lacking the BTB/POZ domain (MamoAF) is a potent vasa activator. In this study, we investigated the genetic interaction between MamoAF and specific transcriptional regulators to gain insight into the transcriptional regulation of germline development. We identified a general transcription factor, TATA box binding protein (TBP)-associated factor 3 (TAF3/BIP2), and a member of the TBP-like proteins, TBP-related factor 2 (TRF2), as new genetic modifiers of MamoAF. In contrast to TRF2, TBP was found to show no genetic interaction with MamoAF, suggesting that Trf2 has a selective function. Therefore, we focused on Trf2 expression and investigated its function in germ cells. We found that Trf2 mRNA, rather than Tbp mRNA, was preferentially expressed in PGCs during embryogenesis. Depletion of TRF2 in PGCs resulted in decreased mRNA expression of vasa. RNA interference-mediated knockdown showed that, while Trf2 is required for maintenance of germ cells, Tbp is needed for their differentiation during oogenesis. Therefore, these results suggest that Trf2 and Tbp expression is differentially regulated in germ cells and that these factors have distinct functions in Drosophila germline development.