HETERODIMERIC DROSOPHILA GAP GENE PROTEIN COMPLEXES ACTING AS TRANSCRIPTIONAL REPRESSORS

The Drosophila gap gene Kruppel (Kr) encodes a transcriptional regulat or. It acts both as an integral part of the Drosophila segmentation ge ne in the early blastoderm and in a variety of tissues and organs at l ater stages of embryogenesis. In transfected tissue culture cells, the Kr protein (KR) was shown to both activate and repress gene expressio n in a concentration-dependent manner when acting from a single bindin g site close to the promoter. Here we show that KR can associate with the transcription factors encoded by the gap genes knirps (kni) and hu nchback (hb) which affect KR-dependent gene expression in Drosophila t issue culture cells. The association of DNA-bound hb protein or free k ni protein with distinct but different regions of KR results in the fo rmation of DNA-bound transcriptional repressor complexes. Our results suggest that individual transcription factors can associate to form pr otein complexes which act as direct repressors of transcription. The i nteractions shown here add an unexpected level of complexity to the co ntrol of gene expression.