Interferon regulatory factor-1 (IRF-1) is a transcription factor exhibiting functional diversity because of its ability to activate transcription from promoters of several IRF-1-dependent genes. It is a modular protein, where the overall structure is not essential for function of its individual domains. A comparison of the mouse and human IRF- I amino acid sequences enabled us to identify a stretch of six amino acids (198-203) within the transactivation domain of mouse IRF-1, (198)MQMDII(203) to be different from that of the human IRF-1, (IPVEVV202)-I-197. This indicated a possible functional significance of the six amino acid stretches in the two IRF-1 molecules. The murine IRF- 1 sequence at 198-203 (MQMDII) was replaced by IPVEVV. Recombinant wild type mouse IRF- 1 with (198)MQMDII(203) and its mutant form with (IPVEVV203)-I-198, expressed as GST-IRF- 1-fusion proteins, showed similar DNA-binding activity. However, ectopic expression of the wild type and mutant IRF-1 in the human embryonic kidney (HEK-293) cells showed the effect of replacement of this region on expression of a few chromosomal genes that are transcriptionally activated by IRF-1 viz. IFN- beta, iNOS, and COX-2 genes. In our study, expression of wild type IRF- 1 activated these genes as judged by RT-PCR but the mutant IRF-1 did not show this effect. Thus, the MQMDII (198-203 a.a.) region of mouse IRF-1 has a functional context in relation to expression of IRF-1-inducible genes. (C) 2003 Elsevier Inc. All rights reserved.