'Don't touch my junk DNA!' says gene signal sequence

Scientists at MIT say they have discovered a mechanism that prevents noncoding DNA from being copied, by pointing the copying in the right direction. 

The Wellcome Trust/Reuters
A computer illustration of the double-helix structure of DNA. Scientists say that they have uncovered a mechanism that prevents cells from copying so-called junk DNA.

Almost all of the human genome is made of noncoding, or "junk" DNA, that is, DNA that usually doesn't get copied and encoded into proteins. 

So when copying DNA, how do cells tell the diference between actual genes and non-coding DNA? 

Transcription begins at regions on the DNA molecule called promoters, sequences located at the beginning of genes that are to be copied. The enzyme that copies DNA, called RNA polymerase, latches on to the promoter and starts unzipping the DNA double helix, spooling out a chain of what will become messenger RNA – mRNA for short – that contains the information of the gene. [Editor's note: The original version of this paragraph used the term "replication" instead of "transcription." DNA replication is a different process. The Monitor regrets the error.]  

But how does the RNA polymerase know which direction to go? Until now, scientists didn't know. But in research published in the current issue of the scientific journal Nature, MIT biologists say they have discovered the mechanism that points transcription in the right direction

In all living things except bacteria, the RNA polymerase continues unzipping the DNA until it reaches a stop signal, at which point it stops copying and begins adding a chain of adenine bases to the pre-mRNA molecule, usually a couple hundred links long. This "poly-A" tail protects the mRNA as it exits the nucleus and travels to the ribosome, where the molecule's information is synthesized into proteins.

By sequencing mRNA of mouse embryonic stem cells, the researchers found that the signal sequences for creating poly-A tails – a process known as polyadenylation – are also prevalent "upstream" from the promoter. An RNA polymerase that encounters these sequences will chop up its pre-mRNA. Sequences of DNA that are to be coded into genes, by contrast, have a low density of polyadenylation signal sequences.

The researchers also found that the polyadenylation signal sequences are more likely to be ignored when they appear within coding sequences, thanks to a tiny protein complex called U1 snRNP. When U1 snRNP binds to an RNA polymerase, polyadenylation is supressed. The researchers discovered that binding sites for U1 snRNP are more prevalent in coding sequences than noncoding ones.

“Once you see some data like this, it raises many more questions to be investigated, which I’m hoping will lead us to deeper insights into how our cells carry out their normal functions and how they change in malignancy,” says Phillip Sharp, a professor at MIT's Koch Institute for Integrative Cancer Research and a co-author of the study, in a statement.

You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.