Banks J A, Masson P, Fedoroff N
Department of Embryology, Carnegie Institution of Washington, Baltimore, Maryland 21210.
Genes Dev. 1988 Nov;2(11):1364-80. doi: 10.1101/gad.2.11.1364.
The maize Suppressor-mutator (Spm) element can exist in one of three heritable forms: (1) a stably active form, (2) a stably inactive form, termed cryptic, and (3) a labile form, here termed programmable, in which the element exhibits one of a variety of heritable developmental programs of expression. Active elements are transcribed and are hypomethylated at sites upstream of the transcription start site, whereas inactive elements are transcriptionally silent and largely methylated at the upstream sites. Active (both stable and programmable), inactive programmable, and cryptic elements are unmethylated, partially methylated, and fully methylated, respectively, at sites within an 0.35-kb 80% G + C region just downstream from the transcription start site. An active Spm element in a genome with a cryptic element promotes its partial demethylation but not its transcriptional activation. In contrast, a trans-acting Spm promotes extensive demethylation and transcriptional activation of an inactive programmable element, as well as its heritable reactivation. These observations define the molecular components of the Spm element's developmental regulatory mechanism. We discuss their general relevance to the developmental regulation of gene expression.
玉米抑制-突变体（Spm）元件可存在于三种可遗传形式之一：（1）稳定活性形式；（2）稳定无活性形式，称为隐性形式；（3）不稳定形式，此处称为可编程形式，其中该元件表现出多种可遗传的表达发育程序之一。活性元件可转录，且在转录起始位点上游区域低甲基化，而无活性元件转录沉默，且在上游位点大多甲基化。活性（包括稳定型和可编程型）、无活性可编程型和隐性元件在转录起始位点下游一个0.35 kb、G + C含量为80%的区域内的位点分别为未甲基化、部分甲基化和完全甲基化。基因组中带有隐性元件的活性Spm元件可促进其部分去甲基化，但不会促进其转录激活。相反，反式作用的Spm可促进无活性可编程元件的广泛去甲基化和转录激活，以及其可遗传的重新激活。这些观察结果确定了Spm元件发育调控机制的分子组成部分。我们讨论了它们与基因表达发育调控的一般相关性。
