据悉，一个人的全部DNA中高达95％的DNA被认为是垃圾DNA。为了防止这些进化过程中产生的废物重排染色体并导致疾病，现代染色体生物学理论认为，必然存在一些能够使它们沉默的天然机制。
RNA沉默机制能通过破坏携带DNA指令的mRNA分子来使基因表达沉默。两年前，有研究表明RNA沉默机器在形成异染色质——无转录活性的非活动染色质——的过程中是必需的，这也暗示出生物学中存在的新“规则”。但Texas A&M大学和Oregon大学的研究人员却持不同意见。在发表在最新一期Science上的文章中，他们打破了以往的“规则”。
Texas A&M 大学生物学副教授Rodolfo Aramayo认为，该研究将有助于人们对染色体结构复杂性的理解，进而在基因治疗中发挥重要作用。由于大部分先天缺陷是由染色体异常造成的，因此了解染色体的机构及其如何运作将非常重要。
在对脉胞菌（Neurospora crassa）的一项研究中，Rodolfo Aramayo及其同事构建出一种突变的细胞，这种细胞不能制造出RNA沉默机制所需的关键成份，结果他们发现这种细胞的异染色质能正常形成。
Aramayo说，“我们可以将染色体想象成一条高速路，为了正常运行，高速路的一些区段需要照明，也就是说DNA必须被表达；而其它区段则不需要照明，即DNA不需要表达，或这些区段必须被异染色质化。......有一些士兵负责关掉高速路上的某些灯。依据这个规则，所有人都认为这些‘士兵’就是RNA沉默机器。但我们的研究表明，当我们去掉这些沉默机器的‘士兵’时，还有其它‘人’仍然能将灯关掉！我们不知道这个‘关灯人’到底是谁，但我们正在努力找出他。”
研究表明，细胞在进化过程中会产生出多种不同的通路去完成同一工作。这一点非常重要，因为人类细胞也可能有相似的机制。
进一步掌握染色体生物学的细节是进行基因治疗的基础。Aramayo说，“了解正常的染色体生物学并非一时好奇，而是战胜疾病必须作的事情......不了解染色体的工作原理，我们永远不能实现基因疗法。希望在将来我们能通过引入基因来纠正遗传错误，从而帮助人们战胜疾病。”他又补充说，“即使我们进行的是基础性的研究，但对整个社会的意义是重大的。”
Function and properties of silencing suppressor proteins
Researchers look into components of RNA silencing machinery
COLLEGE STATION, July 27, 2004 - Up to 95 percent of a person's DNA is believed to be junk DNA. In order to prevent these relics of evolution from rearranging chromosomes and causing disease, natural mechanisms exist to silence them, according to contemporary theories of chromosome biology.
The RNA silencing machinery silences gene expression, by destroying RNA, a molecule that carries out DNA's instructions. Two years ago, components of the RNA silencing machinery were shown to be absolutely required for forming heterochromatin, a chromatin state that silences DNA, suggesting a new rule in biology. But researchers from Texas A&M University and the University of Oregon disagree.
They broke that rule in reporting their findings in the July 25 issue of Science magazine. Funding for the research is supported by grants from the National Institutes of Health.
Knowledge gained from their research will enable scientists to better understand the complexities of chromosome biology, which ultimately will lead to conquering disease by the means of gene therapy, says Rodolfo Aramayo, associate professor of biology at Texas A&M University, who specializes in genetics and studies the biology of meiotic chromosomes. Since the majority of birth defects are caused by chromosomal abnormalities, he says it is absolutely fundamental to understand how chromosomes work.
"Understanding normal chromosome biology is more than a curious scientific endeavor," says Aramayo. "It is a must if we ever are to conquer disease."
In a study with Neurospora crassa, a mold, Aramayo and his colleagues created mutant cells that cannot produce any key components of the RNA silencing mechanism, and discovered that heterochromatin formed just fine.
"Think about a chromosome as being a highway," says Aramayo. "In order to function properly, some sections of the highway must have illumination, which means DNA must be expressed, whereas other sections must not have illumination, which means DNA must not be expressed, or that the region must be heterochromatic.
"Some soldiers carry the duty of turning off certain lights along the highway. Everybody thought these soldiers belong to the RNA silencing machinery, as the rule says. But what our research has shown is that, when we killed these RNA silencing machinery soldiers, someone else is still turning the lights off! We don't know who did it, but we're on our way to find out."
"What we have shown," Aramayo says, "is that cells have evolved more than one way to do the same thing. And this is important, because similar mechanisms might be present in human cells."
In regard to the growing knowledge base in understanding chromosome biology for doing gene therapy, Aramayo says, "Without understanding how chromosomes work, we will never be able to perform gene therapy. Hopefully in the future, we are going to be able to correct genetic mistakes by introducing genes and help people overcome these problems."
Aramayo adds, "Even though we are doing basic research, the implications for society in general are broad."
