| The Yin Yang-1 (YY1) gene encodes a ubiquitous, conserved, and multi-functional zinc finger transcription factor. YY1 has been shown to play a regulatory role in initiation, activation, and repression of transcription for a wide variety of genes required for cell growth, development, differentiation, tumor suppression, and viral gene regulation. YY1 binds to the histone alpha element located within the coding region of the replication-dependent histone genes, which is required for the correct up-regulation of these genes in the cell cycle. To determine if YY1 is sufficient for the formation of the histone alpha complex, a YY1 baculovirus was designed and expressed. Using the recombinant YY1, electrophoretic mobility shift assays indicate that purified YY1 does not bind to the histone regulatory sequence, but alpha complex formation does occur upon column fraction addback assays.;Although YY1 does not bind to histone genes as an isolated protein, but rather as part of a multi-unit complex, fluorescent in-situ hybridization of the histone gene clusters confirms the co-localization of YY1 and the histone genes. Because histone gene transcription is tightly regulated in the cell cycle, the subcellular localization of YY1 was examined to determine if a change in cellular localization during the cell cycle plays a role in the regulation of YY1's activities. After mitotic shake-off, the localization of YY1 was examined in synchronous Chinese hamster ovary cells progressing from mitosis into S phase. Using fluorescent deconvolution microscopy, the subcellular localization of YY1 changes from cytoplasmic to nuclear upon S phase entry. The changes in localization of YY1 correlate well with the up-regulation of the replication-dependent histone genes at the G1/S boundry of the cell cycle, and subsequent down-regulation of these genes later in S phase. Interestingly, YY1 is observed at or proximal to the histone gene clusters in the nucleus at all times examined. At times during the cell cycle when the subcellular localization of YY1 is primarily cytoplasmic, all detectable YY1 in the nucleus is localized near the histone gene clusters. These findings strongly support the hypothesis that YY1 plays a major role in the regulation of the replication-dependent histone gene family. |
