Xu T, Purcell M, Zucchi P, Helentjaris T, Bogorad L
Department of Cellular and Molecular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2001 Feb 27;98(5):2295-300. doi: 10.1073/pnas.041610098.
The genes rbcS and rbcL encode, respectively, the small and large subunits of the photosynthetic carbon dioxide fixation enzyme ribulose bisphosphate carboxylase/oxygenase. There is a single rbcL gene in each chloroplast chromosome; a family of rbcS genes is located in the nuclear genome. These two genes are not expressed in mesophyll cells but are in adjacent bundle-sheath cells of leaves of the C4 plant Zea mays. Two regions of the maize gene rbcS-m3 are required for suppressing expression in mesophyll cells. One region is just beyond the translation termination site in the 3' region, and the other is several hundred base pairs upstream of the transcription start site. A binding site for a protein with limited homology to the viral, yeast, and mammalian transcription repressor-activator YY1 (Yin-Yang I), has now been identified in the 3' region. A maize gene for a protein with zinc fingers homologous to those of YY1 has been isolated, characterized, and expressed in Escherichia coli. The gene is designated trm1 (transcription repressor-maize 1). The protein TRM1 binds to the YY1-like site and, in addition, TRM1 binds to two sequence regions in the 5' region of the gene that have no homology to the YY1 site. Mutagenesis or deletion of any of these three sequences eliminates repression of rbcS-m3 reporter genes in mesophyll cells.
基因rbcS和rbcL分别编码光合二氧化碳固定酶核酮糖二磷酸羧化酶/加氧酶的小亚基和大亚基。每个叶绿体染色体中有一个rbcL基因；rbcS基因家族位于核基因组中。这两个基因在叶肉细胞中不表达，但在C4植物玉米叶片相邻的维管束鞘细胞中表达。玉米基因rbcS - m3的两个区域是抑制叶肉细胞中表达所必需的。一个区域就在3'区域的翻译终止位点之后，另一个区域在转录起始位点上游几百个碱基对处。现已在3'区域鉴定出一个与病毒、酵母和哺乳动物转录抑制激活因子YY1（阴阳1）具有有限同源性的蛋白质结合位点。已分离、鉴定出一个与YY1锌指结构同源的玉米蛋白质基因，并在大肠杆菌中表达。该基因被命名为trm1（转录抑制因子 - 玉米1）。蛋白质TRM1与YY1样位点结合，此外，TRM1还与该基因5'区域中两个与YY1位点无同源性的序列区域结合。对这三个序列中的任何一个进行诱变或缺失都会消除叶肉细胞中rbcS - m3报告基因的抑制作用。
