Mechanism integrating regulation of DNA replication with specific gene expression during the course of cell differentiation

大鼠醛缩酶B(A LDB)基因启动子区域在肝脏特异性转录中起重要作用。此外，在不转录aldB基因的细胞中，这一DNA区域被证明是DNA复制的起始区。为了了解启动子/起始区的功能切换在细胞分化过程中的意义，我们试图确定启动DNA复制所需的顺式元件，并表征它们的作用。启动子/起始区具有一组重叠的用于转录和复制的顺式元件。在活跃转录AldB基因的细胞中，复制从启动子区域以外的区域开始。其次，复制顺式元件以细胞周期依赖的方式结合蛋白质。与核基质结合的DNA区域在表达和不表达AldB的细胞中不同，表明染色体结构域不同。在AldB非表达细胞中，启动子/起始区与核基质相关，而在AldB表达细胞中不与启动子/起始区相关。这些结果并不反对我们的模型，即染色体上的一系列激发起始点与转录基因的模式密切相关，并且起始点的位置变化与细胞分化过程中特定基因表达的变化有关。为了确定上述可能性，应在围绕AldB启动子/起源的广泛染色体区域内，相对于活跃转录的基因的分布确定激发起始点的位置。同样有趣的是，复制起点与核基质的关联会导致染色体结构域的改变，从而导致转录调控。

Promoter region of the rat aldolase B (A ldB) gene has an important role in liver-specific transcription. In addition, this DNA region has been shown to acts as an initiation region of DNA replication in cells that do not transcribe the AldB gene. In order to know significance of functional switch of this promoter/origin region during cell differentiation, we tried to determine cis-elements required for initiation of DNA replication and to characterize their roles.We found that, firstly. the promoter/origin region has an overlapping set of cis-elements for transcription and replication. In cells in which the AldB gene is actively transcribed, repliction initiates from a region other than the promoter region. Secondly, cis-elements for replication bind proteins in a cell cycle-dependent manner. Thirdly, DNA regions that associates with nuclear matrix differed between AldB-expressing and non-expressing cells, indicating different architecture of chromosomal domain. The promoter/origin region in AldB non-expressing cells, the origin region associates with nuclear matrix, while it does not in AldB-expressing cells.These results do not argue against our model where an array of firing origins on the chromosome tightly links the pattern of genes being transcribed, and the positional change of the firing origins are involved in changes of specific gene expression during cell differentiation. To ascertain the above possibility, positioning of firing origins should be determined within a broad range of chromosomal region encompassing the AldB promoter/origin, with respect to distribution of genes that are actively transcribed. It is also interesting to consider that association of replication origins with nuclear matrix causes alteration of chromosomal domains that leads to transcriptional regulation.