Polycomb transcriptional silencing machinery is implicated in the maintenance of precursor fates, but how this repression is reversed to allow cell differentiation is unknown. Here we show that testis-specific TAF (TBP-associated factor) homologs required for terminal differentiation of male germ cells may activate target gene expression in part by counteracting repression by Polycomb. Chromatin immunoprecipitation revealed that testis TAFs bind to target promoters, reduce Polycomb binding, and promote local accumulation of H3K4me3, a mark of Trithorax action. Testis TAFs also promoted relocalization of Polycomb Repression Complex 1 components to the nucleolus in spermatocytes, implicating subnuclear architecture in the regulation of terminal differentiation.

Alternate forms of the PolII transcription initiation machinery have been proposed to play a role in selective activation of cell-type-specific gene expression programs during cellular differentiation. The cannonball(can) gene of Drosophila encodes a homolog of a TBP-associated factor (dTAF5) protein expressed only in spermatocytes, where it is required for normal transcription of genes required for spermatid differentiation. We show that Drosophila primary spermatocytes also express four additional tissue-specific TAFs: nht (homolog of dTAF4), mia (homolog of dTAF6), sa (homolog of dTAF8) and rye (homolog of dTAF12). Mutations in nht, mia and sa have similar effects in primary spermatocytes on transcription of several target genes involved in spermatid differentiation, and cause the same phenotypes as mutations in can, blocking both meiotic cell cycle progression and spermatid differentiation. The nht, mia, sa and rye proteins contain histone fold domain dimerization motifs. The nht and rye proteins interact structurally when co-expressed in bacteria, similarly to their generally expressed homologs TAF4 and TAF12,which heterodimerize. Strikingly, the structural interaction is tissue specific: nht did not interact with dTAF12 and dTAF4 did not interact with rye in a bacterial co-expression assay. We propose that the products of the five Drosophila genes encoding testis TAF homologs collaborate in an alternative TAF-containing protein complex to regulate a testis-specific gene expression program in primary spermatocytes required for terminal differentiation of male germ cells.

Alternate forms of the general transcription machinery have been described in several tissues or cell types. However, the role of tissue-specific TBP-associated factors (TAF II s) and other tissue-specific transcription components in regulating differential gene expression during development was not clear. Here we show that the cannonball gene of Drosophila encodes a cell type-specific homolog of a more ubiquitously expressed component of the general transcription factor TFIID. cannonball is required in vivo for high level transcription of a set of stage-and tissue-specific target genes during male gametogenesis. Regulation of transcription by cannonball is absolutely required for spermatogenesis, as null mutations block meiotic cell cycle progression and result in a complete failure of spermatid differentiation. Our results demonstrate that cell type-specific TAF II s play an important role in developmental regulation of gene expression.

A robust developmentally regulated and cell type specific transcriptional programme is activated in primary spermatocytes in preparation for differentiation of the male gametes during spermatogenesis. Work in Drosophila is beginning to reveal the genetic networks that regulate this gene expression. The Drosophila aly-class meiotic arrest loci are essential for activation of transcription of many differentiation-specific genes, as well as several genes important for meiotic cell cycle progression,thus linking meiotic cell cycle progression to cellular differentiation during spermatogenesis. The three previously described aly-class proteins(aly, comr and achi/vis) form a complex and are associated with chromatin in primary spermatocytes. We identify, clone and characterize a new aly-class meiotic arrest gene, matotopetli (topi), which encodes a testis-specific Zn-finger protein that physically interacts with Comr. The topimutant phenotype is most like achi/vis in that topi function is not required for the nuclear localization of Aly or Comr, but is required for their accumulation on chromatin. Most target genes in the transcriptional programme depend on both topi and achi/vis; however, a small subset of target genes are differentially sensitive to loss of topior achi/vis, suggesting that these aly-class predicted DNA binding proteins can act independently in some contexts.

Mitofusins comprise a family of evolutionarily conserved, nuclear encoded mitochondrial guanosine triphoshatases that control mitochondrial fusion and morphology. The fuzzy onions (fzo) and Drosophila mitofusin (dmfn) genes, which encode the only Mitofusin homologs in Drosophila are differentially expressed during development. Dmfn-mRNA was widely expressed during embryogenesis accumulating in the mesoderm and endoderm during gut development, during oogenesis with transcripts maternally deposited into the early embryo and in the male germ line, where dmfn-mRNA was expressed in spermatogonia, spermatocytes and early spermatids. In contrast, expression of the fzo was tightly restricted to the male germ line, with mRNA accumulation in spermatocytes and early spermatids. In addition, expression of dmfn and fzo in the same cell type, primary spermatocytes, was under control of different regulatory mechanisms.