A soluble transcription factor, Oct-1, is also found in the insoluble nuclear matrix and possesses silencing activity in its alanine-rich domain.

Expression of the human thyrotropin beta (hTSHbeta) gene is restricted to thyrotrophs, at least in part, by silencing. Using transient-transfection assays, we have localized a silencer element to a region between -128 and -480 bp upstream of the transcription initiation site. The silencing activity was overcome in a thyrotroph-specific manner by an unknown enhancer located in the sequences at -approximately 10000 to -1200 bp. The ubiquitous POU homeodomain protein Oct-1 recognized the A/T-rich silencer element at multiple sites in gel mobility shift assays and in vitro footprinting analyses. The silencing activity of Oct-1 was localized in its C-terminal alanine-rich domain, suggesting that Oct-1 plays a role in silencing of the hTSHbeta promoter. Further, a significant fraction of Oct-1 was shown to be associated with the nuclear matrix, and the hTSHbeta silencer region was tethered to a nuclear matrix of human cells in vivo, suggesting a possible role of the Oct-1-hTSHbeta silencer region interaction in chromatin organization.