烟草MARs的分离及其功能分析

从烟草基因组中克隆到两条新的MAR片段（M14和M17），序列分析表明，它们具有90%AT-box,A-box,T-box，碱基非配对区域，拓扑异构酶Ⅱ识别位点，弯曲DNA序列，复制起始序列和ATATTT等典型的MAR序列特征,并与原有MAR序列的特征不同。将它们分别构建到植物表达载体pCAMBIA2301 GUS基因（uidA）表达盒一侧及两侧，通过农杆菌介导转化烟草。组织化学染色法定性检测GUS活性表明，带有M14和M17的uidA基因在转基因烟草中稳定表达。GUS活性的定量检测表明，表达载体上uidA基因一端或两端连接有MAR的转化烟草中,GUS的表达水平与对照相比都有了明显提高，而uidA基因两侧连有MAR的载体提高表达水平的效果优于一端连有MAR的载体，可使GUS活性增强3.14倍,但不同转化个体之间表达水平的差异仍然明显。上述结果表明，所得DNA序列为两条新的MAR片段，并且具有提高转基因表达水平的功能。

Isolation and Functional Analysis of Tobacco MARs

Two new MAR segments (M14 and M17) were cloned from tobacco genome. Both of the sequences contained several typical consensus sequences of MARs, which were different from the original MAR sequence, such as 90%AT-box, A-box, T-box, the base unpairing regions(BUR), autonomously replicating sequences (ARS), the consensus sequence for topoisomerase II, MAR recognition sequence (MRS), origin of replication (ORI), curved DNA motifs and ATATTT et al. To investigate the effects of these two sequences on gene expression in transgenic plants, 3 plant expression vectors were constructed with uidA gene coding beta-glucuronidase (GUS) which were flanked on one side and on both sides by the MARs we obtained. These plant expression vectors with one or two MARs were transformed into tobaccos via Agrobacterium-mediated transformation method, with the plant expression vector pCAMBIA2301 without MAR and wild type tobacco as controls. GUS histochemical staining results showed that the uidA gene expressed stably in transgenic tobaccos. Quantitative detection of GUS activity showed that the MARs could increase GUS expression levels in vivo in contrast to the controls, wherever they were flanked on one side or both sides of uidA gene. The vector ligated with MARs in the same direction on both sides of uidA could increase the GUS expression level much better than both vectors which just ligated with single MARs on one side. The former one increased the average GUS activity for 3.14 folds, but 1.56 and 2.43 folds for the latter two vectors with single MARs respectively contrasting to the pCAMBIA2301 control. But the expression differences among individual transformants were still obvious. Therefore, it was concluded that the DNA sequences we obtained in this experiment were two novel MARs and could enhance gene expression in vivo. In the meanwhile, although the numbers of the MARs typical motifs in M14 were more than in M17, especially the 90% AT box which had been considered to be the highest correlative motif with binding strength in vitro, the enhancement of gene expression was lower yet, which implied no correlation between improvement of gene expression and binding strength between MARs and nuclear matrix in vitro.