Copertino D W, Hallick R B
Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721.
EMBO J. 1991 Feb;10(2):433-42. doi: 10.1002/j.1460-2075.1991.tb07965.x.
The psbF gene of chloroplast DNAs encodes the beta-subunit of cytochrome b-559 of the photosystem II reaction center. The psbF locus of Euglena gracilis chloroplast DNA has an unusual 1042 nt group II intron that appears to be formed from the insertion of one group II intron into structural domain V of a second group II intron. Using both direct primer extension cDNA sequencing and cDNA cloning and sequencing, we have determined that a 618 nt internal intron is first excised from the 1042 nt intron of psbF pre-mRNA, resulting in a partially spliced pre-mRNA containing a 424 nt group II intron with a spliced domain V. The 424 nt intron is then removed to yield the mature psbF mRNA. Therefore, the 1042 nt intron of psbF is a group II intron within another group II intron. We use the term 'twintron' to define this new type of genetic element. Intermediates in the splicing pathway were detected by northern hybridization. Splicing of both the internal and external introns occurs via lariat intermediates. Twintron splicing was found to proceed by a sequential pathway, the internal intron being removed prior to the excision of the external intron. A possible mechanism for twintron formation by intron transposition is discussed.
叶绿体DNA的psbF基因编码光系统II反应中心细胞色素b - 559的β亚基。纤细裸藻叶绿体DNA的psbF基因座有一个异常的1042 nt II类内含子，它似乎是由一个II类内含子插入到另一个II类内含子的结构域V中形成的。通过直接引物延伸cDNA测序以及cDNA克隆和测序，我们确定一个618 nt的内部内含子首先从psbF前体mRNA的1042 nt内含子中切除，产生一个部分剪接的前体mRNA，其中包含一个带有剪接结构域V的424 nt II类内含子。然后424 nt的内含子被去除，产生成熟的psbF mRNA。因此，psbF的1042 nt内含子是另一个II类内含子中的II类内含子。我们用“双内含子”这个术语来定义这种新型遗传元件。通过Northern杂交检测到剪接途径中的中间体。内部和外部内含子的剪接均通过套索中间体进行。发现双内含子剪接通过一个顺序途径进行，内部内含子在外部内含子切除之前被去除。讨论了通过内含子转座形成双内含子的一种可能机制。
