Transcription is tightly regulated by proteins called transcription factors. These proteins bind to specific DNA sequences in the gene's promoter region and either recruit or block the enzymes needed to copy that gene into mRNA.
For this study, the researchers adapted the CRISPR system to act as a transcription factor. First, they modified the usual CRISPR protein, known as Cas9, so that it could no longer snip DNA after binding to it. They also added to the protein a segment that activates or represses gene expression by modulating the cell's transcriptional machinery.
To get Cas9 to the right place, the researchers also delivered to the target cells a gene for an RNA guide that corresponds to a DNA sequence on the promoter of the gene they want to activate.
The researchers showed that once the RNA guide and the Cas9 protein join together inside the target cell, they accurately target the correct gene and turn on transcription. To their surprise, they found that the same Cas9 complex could also be used to block gene transcription if targeted to a different part of the gene.
"This is nice in that it allows you do to positive and negative regulation with the same protein, but with different guide RNAs targeted to different positions in the promoter," Lu says.
'A lot of flexibility'
The new system should be much easier to use than two other recently developed transcription-control systems based on DNA-binding proteins known as zinc fingers and transcription activator-like effector nucleases (TALENs), Lu says. Although they are effective, designing and assembling the proteins is time-consuming and expensive.
"There's a lot of flexibility with CRISPR, and it really comes from the fact that you don't have to spend any more time doing protein engineering. You can just change the nucleic a
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology