Regulation of CRISPR-associated (cas) genes in an archaeon

CRISPR-cas systems provide defense against selfish genetic elements, such as viruses and plasmids to most archaea and some bacteria. However, research into the regulation of those systems has so far focused exclusively on bacteria, and little is known about the factors that activate the system in archaea. Previous work on bacteria shows that high bacterial density and quorum sensing signals cause induction of the CRISPR-Cas system.

Our study aims to better understand the factors affecting acquisition of new immune memory (CRISPR spacers) in archaea, known as “adaptation” and uses as a model the CRISPR-Cas system of type I-B of the archaeon Haloferax volcanii. We first showed that the levels of expression of the cas genes involved in the adaptation phase vary between the different growth stages of Hfx. volcanii, and increase during the archaeal late-logarithmic stage when archaeal density is higher.

We then incubated mid-logarithmic archaea, with a conditioned medium derived from late-logarithmic grown archaea. The results showed higher expression of the adaptation cas genes among the archaea incubated with the conditioned medium compared to the control archaea grown in the control medium. Our results suggest the presence of a signal compound in the medium that causes the induction of the adaptation cas genes, possibly a quorum sensing molecule released to the medium by Hfx. volcanii. We also show the effects of membrane stress, which simulates the injection of viral DNA, on the expression of cas genes in Hfx. volcanii.