The maternal effect gene, abnormal oocyte (abo), of Drosophila melanogaster encodes a specific negative regulator of histones

The abnormal oocyte (abo) gene of Drosophila melanogaster is a peculiar mat ernal effect gene whose mutations cause a maternal-effect lethality that ca n be rescued by specific regions of heterochromatin during early embryogene sis. Here we show that abo encodes an evolutionary conserved chromosomal pr otein that localizes exclusively to the histone gene cluster and binds to t he regulatory regions of such genes. We also show a significant increase of histone transcripts in eggs of abo mutant mothers and a partial rescue of the abo maternal-effect defect by deficiencies of the histone gene cluster. On the basis of these results, we suggest that the Abo protein functions s pecifically as a negative regulator of histone transcription and propose a molecular model to account for the ability of heterochromatin to partially rescue the abo maternal-effect defect. Our model proposes that increased do ses of specific heterochromatic regions titrate out abnormally high levels of histories present in embryos from mutant abo mothers and that a balanced pool of histones is critical for normal embryogenesis in Drosophila.