A Y-box protein is a suppressor factor that decreases thyrotropin receptor gene expression.

The decanucleotides in a tandem repeat, -162 to -140 bp, are suppressor elements that decrease TSH receptor (TSHR) gene expression by different mechanisms. A factor(s) interacting with the 3'-decanucleotide compete for proteins that bind the cAMP response element, -139 to -132 bp, a constitutive enhancer necessary for efficient TSHR expression. The 5'-decanucleotide is in a CT-rich, S1 nuclease-sensitive region of the promoter; its suppressor activity has been related to its ability to bind a nonthyroid-specific protein to its coding strand. In this report we clone a complementary DNA encoding a single strand DNA-binding protein that forms a specific protein-DNA complex with the coding strand of the 5'- but not the 3'-decanucleotide and not with the 5'-decanucleotide noncoding or double strand. We show, by cotransfection with TSHR promoter-chloramphenicol acetyltransferase chimeras, that the protein is a suppressor that regulates the function of the 5'- but not the 3'-decanucleotide. The protein is a Y-box protein that was previously cloned as an enhancer factor from the rat liver; it is, however, 95% identical to human YB-1, which suppresses major histocompatibility class II gene expression, and to human nuclease-sensitive element protein-1, a Y-box protein identified by its ability to bind single strand, CT-rich, nuclease-sensitive elements of genes that, like the TSHR, have GC-rich promoters. Unexpectedly, the Y-box protein binds two other sites in the minimal TSHR promoter in a single strand-specific fashion and acts a suppressor at each of these sites. One is associated with the insulin response element of the minimal TSHR promoter and is not in an overtly CT-rich region. The other is located 3' to the cAMP response element in a region termed the S-box, -120 to -113 bp, because of its homology to the S-box of the major histocompatibility class II promoter; this site is in a CT-rich area and, as in the class II promoter, is linked to cAMP-induced gene suppression. A conserved CCTC sequence in each site is important for the binding and suppressor function of the Y-box protein.