A SOLUBLE TRANSCRIPTION FACTOR, OCT-1, IS ALSO FOUND IN THE INSOLUBLENUCLEAR MATRIX AND POSSESSES SILENCING ACTIVITY IN ITS ALANINE-RICH DOMAIN

Expression of the human thyrotropin beta (hTSH beta) gene is restricte d to thyrotrophs, at least in part, by silencing, Using transient-tran sfection assays, we have localized a silencer element to a region betw een -128 and -480 bp upstream of the transcription initiation site. Th e silencing activity was overcome in a thyrotroph-specific manner by a n unknown enhancer located in the sequences at -similar to 10000 to -1 200 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 w as localized in its C-terminal alanine-rich domain, suggesting that Oc t-1 plays a role in silencing of the hTSH beta promoter, Further, a si gnificant fraction of Oct-1 was shown to be associated with the nuclea r matrix, and the hTSH beta silencer region was tethered to a nuclear matrix of human cells in vivo, suggesting a possible role of the Oct-1 -hTSH beta silencer region interaction in chromatin organization.