OVO transcription factors function antagonistically in the Drosophila female germline

OVO controls germline and epidermis differentiation in flies and mice. In t he Drosophila germline, alternative OVO-B and OVO-A isoforms have a common DNA-binding domain, but different N-termini, We show that these isoforms ar e transcription factors with opposite regulatory activities. Using yeast on e-hybrid assays, we identified a strong activation domain within a common r egion and a counteracting repression domain within the OVO-Aspecific region . In flies, OVO-B positively regulated the ovarian tumor promoter, while OV O-A was a negative regulator of the ovarian turner and eve promoters. OVO-B isoforms supplied ovo(+) function in the female germline and epidermis, wh ile OVO-A isoforms had dominant-negative activity in both tissues, Moreover , elevated expression of OVO-A resulted in maternal-effect lethality while the absence of OVO-A resulted in maternal-effect sterility. Our data indica te that tight regulation of antagonistic OVO-B and OVO-A isoforms is critic al for germline formation and differentiation.