Dimerization and the control of transcription by Krüppel.

Krüppel (KR), a Drosophila zinc finger-type transcription factor, can both activate and repress gene expression through interaction with a single DNA-binding site. The opposite regulatory effects of KR are concentration-dependent, and they require distinct portions of KR such as the N-terminal region for activation and the C-terminal region for repression. Here we show that KR is able to form homodimers through sequences located within the C terminus. When these sequences were fused to separated functional parts of the yeast transcription factor GAL4, they reconstituted a functional transcriptional activator on dimerization in vivo. Our results suggest that the KR monomer is a transcriptional activator. At higher concentration KR forms a homodimer and becomes a repressor that functions through the same target sequences as the activator.