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研究人员发现，一组先前被认为没有功能的RNA分子事实上可能有着保护生殖细胞免于自我毁灭的作用。这一发现发表于11月17日出版的《细胞》杂志。

此项发现的关键是基因表达的基本过程。当一个基因开始产生一个蛋白分子，组成该基因的DNA双链解链。第一条链生成一种叫信使RNA的分子，信使RNA起到蛋白质模板的作用。生物学家称第一条链为“有义”链或“编码”链。虽然第二条链不编码蛋白，有时它可能产生“反义”RNA分子，RNA分子和信使RNA分子或有义RNA分子互补。人们检测到很多基因有反义RNA，但大多认为这只是遗传学上的怪僻现象。

通过研究面包师所用的普通的酵母，Whitehead生化研究所的博士后Cintia Hongay和麻省理工学院的Gerald Fink教授发现，在称作IME4的基因中，反义RNA阻断了有义RNA。换句话说，该基因自己剥夺了合成蛋白质的能力。

“这是高等细胞中反义RNA所具特殊功能的第一个例子”，文章资深作者Fink说：“这一发现为我们指向真核细胞基因调控过程的全新领域。”

正义－反义之间的矛盾有轨可循，类似阴阳相克。当酵母细胞的生存环境很好，有着充足的营养的时候，细胞开始有丝分裂，也就是，DNA复制，从而每个子细胞获得和母细胞相同的染色体数。然而，当酵母细胞挨饿的时候，作为开关的IME4基因打开了，开始了减数分裂过程。细胞分裂成类似哺乳动物卵子和精子细胞的孢子，孢子的染色体数只有母细胞的一半。酵母孢子的体积比它们的母细胞小，更能适应严酷的环境。

但在一些情况下，按下减数分裂开关可能引发大麻烦。如果一个细胞的每条染色体只有一个拷贝（单倍体细胞）被迫进行减数分裂，后代无法存活。幸好单倍体细胞避免了这样的破坏性减数分裂，因为它们能不断产生IME4反义 RNA，阻断了有义RNA的产生。这样，反义IME4保卫了不能进行有丝分裂的单倍体细胞。

“我们首次发现了高等细胞中反义RNA的这一功能，它不是基因沉默”Hongay说：“事实上，的确是第一次看到的一个基因以这种方式调节自身。”

“多年来，科学家们一直通过检测有义RNA来评价基因组，反义RNA被认为是根本没有意义，”Fink说道。“现在我们发现一个反义RNA调节有义RNA的过程。这一过程可能在哺乳动物生殖细胞中发生。事实上，考虑到这些反义RNA的广泛存在，这些发现可能引导我们最终发现全新的基因调控。”

Hongay现致力于查找酵母基因组中可能受反义RNA调控的其他基因。

来源：http://news.biocompare.com/newsstory.asp?id=160107

【天气突然转冷，忘记添衣服，流着鼻涕翻译的，不容易哦！】

Scientists Discover Role For Dueling RNAs

11/16/2006

Source: Whitehead Institute for Biomedical Research

Researchers have found that a class of RNA molecules, previously thought to have no function, may in fact protect sex cells from self-destructing. These findings will be published in the November 17 issue of the journal Cell.

Central to this discovery is the fundamental process of gene expression. When a gene is ready to produce a protein, the two strands of DNA that comprise the gene unravel. The first strand produces a molecule called messenger RNA, which acts as the protein's template. Biologists call this first strand of DNA the "sense" or "coding" transcript. Even though the other strand doesn't contain a protein recipe, it may also, on occasion, produce an "anti-sense" RNA molecule, one whose sequence is complementary to that of the messenger, or sense, RNA. Antisense RNA has been detected for a number of genes, but is largely considered a genetic oddity.

Using common baker's yeast, Cintia Hongay, a postdoctoral researcher in the lab of Whitehead Member and MIT Professor Gerald Fink, discovered that in the case of a gene called IME4, the antisense RNA blocks the sense RNA. In other words, the gene disables its own ability to make protein.

"This is the first case where a specific function in a higher cell for antisense RNA has been found," says Fink, senior author on the paper. "This points to an entirely new process of gene regulation that we've never seen before in eukaryotic cells."

There is a method to this sense/antisense madness, one that has a kind of yin and yang quality. When conditions around yeast cells are good and rich in nutrients, the cells divide by mitosis--that is, the DNA duplicates so each daughter cell receives exactly the same number of chromosomes as the original cell. However, when the yeast cells are starving, IME4 switches on and activates a process called meiosis. Here, the cells divide into germ-cell spores that, like mammalian egg and sperm cells, have half the number of chromosomes. Yeast spores withstand this harsh environment far more ably than the larger cells from which they originate.

But in some cases, flipping the meiotic switch can be catastrophic. If a cell with only one copy of each chromosome (a haploid cell) is forced into meiosis, the progeny won't survive. Fortunately, such destructive meiotic division is avoided in haploid cells because they continually produce IME4 antisense RNA, blocking the production of sense RNA. Antisense IME4, then, safeguards against meiosis in cells that can't handle it.

"This is the first time that we've found a function for antisense RNA, that is not RNAi, in a higher cell type," says Hongay. "In fact, it's really the first time we've seen a gene regulate itself in this way."

"For years scientists have evaluated genomes by measuring the sense RNA, with antisense transcripts thought to have no meaning at all," says Fink. "Here we've found a process in which antisense RNA regulates sense RNA. This same process may occur in the sex cells of mammals. In fact, considering how widespread these antisense transcripts are, I wouldn't be surprised if these findings eventually lead us to discover an entirely new level of gene regulation."

Hongay is now searching the yeast genome for other genes that might be regulated by antisense RNA.

From http://news.biocompare.com/newsstory.asp?id=160107