Haun William J, Laoueillé-Duprat Sylvia, O'connell Mary J, Spillane Charles, Grossniklaus Ueli, Phillips Allison R, Kaeppler Shawn M, Springer Nathan M
Department of Plant Biology, University of Minnesota, 1445 Gortner Avenue, St Paul, MN 55108, USA.
Plant J. 2007 Jan;49(2):325-37. doi: 10.1111/j.1365-313X.2006.02965.x. Epub 2006 Dec 20.
Imprinted gene expression refers to differential transcription of alleles depending on their parental origin. To date, most examples of imprinted gene expression in plants occur in the triploid endosperm tissue. The Arabidopsis gene MEDEA displays an imprinted pattern of gene expression and has homology to the Drosophila Polycomb group (PcG) protein Enhancer-of-zeste (E(z)). We have tested the allele-specific expression patterns of the three maize E(z)-like genes Mez1, Mez2 and Mez3. The expression of Mez2 and Mez3 is not imprinted, with a bi-allelic pattern of transcription for both genes in both the endosperm and embryonic tissue. In contrast, Mez1 displays a bi-allelic expression pattern in the embryonic tissue, and a mono-allelic expression pattern in the developing endosperm tissue. We demonstrate that mono-allelic expression of the maternal Mez1 allele occurs throughout endosperm development. We have identified a 556 bp differentially methylated region (DMR) located approximately 700 bp 5' of the Mez1 transcription start site. This region is heavily methylated at CpG and CpNpG nucleotides on the non-expressed paternal allele but has low levels of methylation on the expressed maternal allele. Molecular evolutionary analysis indicates that conserved domains of all three Mez genes are under purifying selection. The common imprinted expression of Mez1 and MEDEA, in concert with their likely evolutionary origins, suggests that there may be a requirement for imprinting of at least one E(z)-like gene in angiosperms.
印记基因表达是指等位基因根据其亲本来源进行差异转录。迄今为止，植物中印记基因表达的大多数例子都发生在三倍体胚乳组织中。拟南芥基因MEA表现出印记基因表达模式，并且与果蝇的多梳蛋白组（PcG）蛋白增强子（E（z））具有同源性。我们已经测试了三个玉米E（z）样基因Mez1、Mez2和Mez3的等位基因特异性表达模式。Mez2和Mez3的表达没有印记，这两个基因在胚乳和胚胎组织中均呈现双等位基因转录模式。相比之下，Mez1在胚胎组织中表现出双等位基因表达模式，而在发育中的胚乳组织中表现出单等位基因表达模式。我们证明，母本Mez1等位基因的单等位基因表达在整个胚乳发育过程中都存在。我们已经鉴定出一个556 bp的差异甲基化区域（DMR），位于Mez1转录起始位点上游约700 bp处。该区域在未表达的父本等位基因的CpG和CpNpG核苷酸上高度甲基化，但在表达的母本等位基因上甲基化水平较低。分子进化分析表明，所有三个Mez基因的保守结构域都受到纯化选择。Mez1和MEA共同的印记表达，以及它们可能的进化起源，表明被子植物中可能至少需要一个E（z）样基因进行印记。
