Nucleolar colocalization of TAF1 and testis-specific TAFs during Drosophila spermatogenesis.

In Drosophila, testis-specific TBP-associated factors (tTAFs) predominantly localize to spermatocyte nucleoli and regulate the transcription of genes necessary for spermatocyte entry into meiosis. tTAFs are paralogs of generally expressed TAF subunits of transcription factor IID (TFIID). Our recent observation that the generally expressed TAF1 isoform TAF1-2 is greatly enriched in testes prompted us to explore the functional relationship between general TAFs and tTAFs during spermatogenesis. Analysis by immunofluorescence microscopy revealed that among the general TFIID subunits examined (TATA-box binding protein [TBP], TAF1, TAF4, TAF5, and TAF9), only TAF1 colocalized with the tTAF Mia in spermatocyte nucleoli. Nucleolar localization of TAF1, but not Mia, was disrupted in tTAF mutant flies, and TAF1 dissociated from DNA prior to Mia as spermatocytes entered meiosis. Taken together, our results suggest stepwise assembly of a testis-specific TFIID complex (tTFIID) whereby a TAF1 isoform, presumably TAF1-2, is recruited to a core subassembly of tTAFs in spermatocyte nucleoli.