Interplay between repressing and activating domains defines the transcriptional activity of IRF-1

Interferon regulatory factor-1 (IRF-1) is a transcriptional activator with weak activation capacity. By defining the transcriptional activation domain of IRF-1 we identified two activator fragments located between amino acids 185 and 256 functioning in an additive manner. Another fragment of IRF-1, which has no activator function alone, acts as a strong enhancer element of these activator sequences. This enhancer element resides between the activ ator domains and the C-terminus. In addition, we identified a novel type of inhibitory domain in the N-terminal 60 amino acids of IRF-1 which strongly inhibits its transcriptional activity. Because this fragment is conserved in all interferon regulatory factors, we found similar repression effects i n the corresponding fragments in IRF-2, IRF-3 and interferon consensus sequ ence binding protein (ICSBP/IRF-8). Interestingly, the corresponding sequen ce in p48/IRF-9 is divergent, so that it does not show this inhibitory acti vity. A five-amino-acid sequence distinguishes the p48/IRF-9 N-terminus fro m the homologous parts in other interferon regulatory factors containing th e repressing function. Replacing the diverged amino acids in IRF-1 with the corresponding sequence of p48/IRF-9 resulted in a loss of inhibitory activ ity within IRF-1. The opposing activities within interferon regulatory fact ors may contribute to balanced or tuned regulation of gene activation, depe nding on the promoter context.