How Stat1 mediates constitutive gene expression: a complex of unphosphorylated Stat1 and IRF1 supports transcription of the LMP2 gene.

Analysis of mRNA levels in cells that express or lack signal transducers and activators of transcription 1 (Stat1) reveals that Stat1 mediates the constitutive transcription of many genes. Expression of the low molecular mass polypeptide 2 (LMP2), which requires Stat1, has been studied in detail. The overlapping interferon consensus sequence 2/gamma-interferon-activated sequence (ICS-2/GAS) elements in the LMP2 promoter bind to interferon regulatory factor 1 (IRF1) and Stat1 and are occupied constitutively in vivo. The point mutant of Stat1, Y701F, which does not form dimers involving SH2-phosphotyrosine interactions, binds to the GAS element and supports LMP2 expression. Unphosphorylated Stat1 binds to IRF1 directly and we conclude that this complex uses the ICS-2/GAS element to mediate constitutive LMP2 transcription in vivo. The promoter of the IRF1 gene, which also contains a GAS site but not an adjacent ICS-2 site, is not activated by Stat1 Y701F. The promoters of other genes whose constitutive expression requires Stat1 may also utilize complexes of unphosphorylated Stat1 with IRF1 or other transcription factors.