On September 5, 2017, Nucleic Acids Research published an article “Cmr1 enables efficient RNA and DNA interference of a III-B CRISPR-Cas system by binding to target RNA and crRNA”, reporting the new progress on mechanism study of type III-B CRISPR-Cas systems by Molecular Biology of Archaea team. Dr. Yingjun Li and Yan Zhang, a Ph.D candidate of College of life Science and Technology, are first authors of this paper.

CRISPR–Cas (clustered regularly interspaced short palindromic repeats, CRISPR-associated) systems provide adaptive immunity against invasive nucleic acids guided by CRISPR RNAs (crRNAs) in archaea and bacteria. In the latest classification scheme, CRISPR-Cas systems are classified into two classes with six types among which three main types (Type I, II and III) are widely investigated.The Type II CRISPR/Cas9 systems are currently most used for genome engineering in many different bacteria and eukaryotes. Studying molecular mechanisms of different CRISPR-Cas systems is the foundation of their application and has become one of the hottest research fields in contemporary biology. The type III CRISPR systems are reported to be both RNases and RNA-activated DNases, which are significantly different from other CRISPR systems.

More recently, the native effector complex has been purified from the archaeon and characterized. The ribonucleoprotein complex exhibits RNA-activated ssDNA cleavage activity, these results have been published in an NAR paper entitled “A type III-B CRISPR-Cas effector complex mediating massive target DNA destruction” by W. Han, Y. Li, … Q. She. These data provide solid basis of the investigation into molecular mechanisms of Cmr-a system in the current article.

In the structural models of III-B effector complexes, the 3’-end region of crRNAs is located in the Cmr1 subunit. Here, this study aimed to investigate the functions of Cmr1 in the dual DNA/RNA interference by the S. islandicus Cmr-α. A deletion and two alanine-scanning mutants of Cmr1α were conducted and employed for in vivo genetic analysis on Cmr-α function and for biochemical characterization of Cmr-α complex isolated from each mutant. The results showed that Cmr1α functions as an activation module in Cmr-α to strongly enhance its activity and the protein achieves the activation by strongly binding to both target RNA and crRNA. Furthermore, this study laid an important foundation for the application of type III CRISPR systems.

Links:

https://academic.oup.com/nar/article-lookup/doi/10.1093/nar/gkx791

https://academic.oup.com/nar/article-lookup/doi/10.1093/nar/gkw1274

(By Yingjun Li)