Too many transcription factors: positive and negative interactions.

Eukaryotic transcription factors can be classified into several families on the basis of conserved sequences among their DNA-binding domains. Because of such structural conservation, several different trans-acting factors can often interact with a common binding site. Recent findings reviewed herein indicate that the interaction of different factors with a common target site does not necessarily result in equivalent transcriptional responses. While some factors activate transcription, others that bind to the same site repress this process. The examples described here illustrate an emerging new mode of cellular regulation mediated by closely related but functionally distinct transcription factors that appear to compete for common binding sites.