Posts Tagged “CRISPR”

Nature, mother nature & the famous journal, taught us that every organism has its own defense mechanisms against various predators. For example, the famous antifungal agent (cyclohexamide) is obtained from the bacteria Streptomyces, on the other hand (Penicillin), the antibiotic, comes from the fungus Penicillium.

We all know phages, the nick name of Bacteriophages, the virus-like agents that infect bacteria making it sick.. Well not sick, but only degrade it like any other virus on the planet. As a matter of fact, Bacteria have to develop defense mechanisms against these phages:

1)They can cut their genome with restriction enzymes (endonucleases)

2)They can also undergo changes in their receptors, so the phage goes blind & never find it

3)They can act on the phage itself by making DNA modifications or even repression of their gene expression.

But now we’ll talk about a different defense mechanism (they love to call it: Special Forces). To know it, you’ve to meet CRISPR sequences (clustered, regularly interspaced, short, palindromic repeats). Not crispy, it’s CRISPR. Actually when I first read it, I was totally lost. I knew the meaning of every word separated from the very next. So I checked more & got this from the amazing blog of Tim “Phage Hunter“.

As you’ve read before, they are sequences found in almost 40% of sequenced bacteria & 90% of sequenced archaea. There are already identical repeats which form RNA stem-loops. Between those repeats, researchers found DNA which is similar to that of phages. That means that the bacteria use the RNA interference mechanism (an inhibitory gene expression mechanism).
CRISPR sequences are first transcribed, and then spliced to form small interfering RNA (siRNA), which are complementary to the target mRNA (the phage’s). Once binding achieved, no translation occurs, because they simply cleave it into little pieces.

Bacterial CRISPR is modeled to work as iRNA in eukaryotes

So the array of these sequences is highly useful in determining the bacterial resistance to different phages. Y. pestis (aka Black death) has three CRISPR sequences in its genome. It’s something like acquired immunity, bacteria develop it after the infection of the phage, the survivors of course.

For people On The Run: Bacteria have a complementary sequence of their phages, to capture their RNA, stop the translation process.

Image credits:
Figure shows the role of siRNA in degradation of phage nucleic acids: http://www.phagehunter.org

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