Genome complexity: Explaining Crispr (Introduction)

by David Turell , Wednesday, March 29, 2017, 23:38 (2581 days ago) @ David Turell

The bacterial Crispr defense system is further explained in that it immediately attacks invading viruses:

https://phys.org/news/2017-03-microbes-viruses-fast-response-defense.html

"The Rockefeller University reveals how microbes act quickly to fend off the incoming threat using CRISPR, a bacterial immune system that also serves as a powerful tool for editing genomes.

"The discovery, described in Nature on March 29, answers a long-standing question as to how CRISPR works.

Scientists already knew the basics: When a bacterial cell is invaded by viral DNA, its CRISPR system captures snippets of it and catalogs those DNA pieces. Should the same virus show up again, the system will quickly recognize it.

"'For about a decade, we've known that CRISPR works by acquiring pieces of viral DNA, but it's been a mystery exactly when this key step in CRISPR immunity occurs during an infection," says Luciano A. Marraffini, head of the Laboratory of Bacteriology, who studies CRISPR systems in their native bacteria.

"Experiments in his lab revealed that CRISPR springs into action early on, when the virus is injecting itself into the cell.

"To pinpoint the timing, the research team designed an experiment to stop the viral life cycle at different points. Joshua W. Modell, a postdoc, and Wenyan Jiang, a graduate student, then examined the CRISPR systems to see when and how they acquired spacers from the virus.

"Just any DNA won't do for CRISPR; previous research has shown it favors spacers from the loose ends of DNA. This preference narrows the options, since viral DNA takes linear form only during certain points in the infection; the rest of the time its two ends stick together, creating a circle.

"The team halted the infection at three points. But regardless of when they stopped it, CRISPR continued to acquire spacers, indicating it picks them up at the beginning, around when the virus injects its genome—as a strand—into the cell.

"This timing matters. By taking spacers from the first part of the virus to enter the cell, CRISPR ensures it will attack the virus as soon as it shows up next time. When the researchers altered CRISPR systems to contain spacers that matched sequences at the end of the viral genome, the last part to be injected, the microbes struggled to proliferate. (my bold)

"'It turns out that the CRISPR system is very clever," says Modell. "It takes advantage of a nuance of the viral infection cycle to halt an infection as early as possible.'"

Comment: The Crispr system is a series of enzymes that break down foreign DNA. It is difficult to imagine that this defense system took time to develop. If stepwise there would be only dead bacteria, as the virus attacker would always win. Looks like a saltation to me.