Two types of zinc fingers are required for dimerization of the serendipity delta transcriptional activator

The serendipity (sry) delta zinc finger protein controls bicoid gene expression during Drosophila melanogaster oogenesis. In addition, sry delta mutants display various zygotic phenotypes, ranging from abnormal embryogenesis to sex-biased adult lethality. We report here that sry delta is a sequence-specific transcriptional activator. A single sry delta consensus binding site (SDCS), in either orientation, is sufficient to promote transcription activation in cell culture, and multiple SDCSs mediate a strong synergistic activation, reflecting the cooperativity of sry delta binding to DNA. Further, several lines of evidence strongly suggest that sry delta binds to DNA as a dimer. While each of three point mutations located in the third zinc finger of sry delta drastically reduces its DNA binding affinity, a fourth mutation, located in the N-terminal region of the protein, specifically affects the cooperativity of DNA binding. This mutation reveals the functional importance of a putative Cys2/Cys2 zinc finger motif of a novel type, located outside the DNA binding domain. A systematic deletion analysis shows that interaction between this proposed Cys2/Cys2 motif and a classical Cys2/His2 zinc finger mediates homodimerization, which is required for DNA binding cooperativity.