B. Anand

B. Anand

Department of Biosciences & Bioengineering,
Indian Institute of Technology, Guwahati 781 039
Email: banand@iitg.ernet.in, anandbasub@gmail.com

B Anand, IIT, Guwahati, selected to Associateship in 2015. The focus of Anand’s research group is directed towards addressing the mechanism of RNA centric biological processes such as CRISPR-Cas system and ribosome assembly by employing interdisciplinary approaches.

Session 3A - Inaugural Lecture

Homing mechanism of the CRISPR-Cas adaptive immune system

The CRISPR-Cas system represents genetically encoded prokaryote-specific adaptive defence machinery that limits the genome invasion by mobile genetic elements such as bacteriophages and plasmids by employing an RNA-mediated interference mechanism. This system comprises a non-coding CRISPR array and a CRISPR-associated (cas) operon. The CRISPR array comprises direct repeats of about 3040 nts that are intervened by similarly sized variable DNA referred as spacers that are derived from invading foreign DNA. The cas operon encodes several Cas proteins that together with the CRISPR RNA form a ribonucleoprotein surveillance complex. In this complex, the RNA guides the target recognition by base complementarity, whereas the proteins facilitate the target cleavage.

The cornerstone of this immune system is that it confers heritable adaptive immunity to bacteria by capturing a protospacer fragment from invading foreign DNA, which is later inserted into the leader proximal end of CRIPSR array and serves as immunological memory to recognise recurrent invasions, akin to antibodies in the case of higher organisms. The universally conserved Cas1 and Cas2 form an integration complex that is known to mediate the protospacer invasion into the CRISPR array. However, the mechanism by which this protospacer fragment gets integrated in a directional fashion into the leader proximal end is elusive. The speakers group identified that the leader region abutting the first CRISPR repeat localizes Integration Host Factor (IHF) and Cas1-2 complex in Escherichia coli. IHF binding to the leader region induces bending by about 120°, which in turn engenders the regeneration of the cognate binding site for protospacer-bound Cas1-2 complex and brings it in proximity with the first CRISPR repeat. This appears to guide Cas1-2 complex to orient the protospacer invasion towards the leader-repeat junction thus driving the integration in a polarised fashion. Their work thus illuminates how Cas1-2 complex and IHF coordinate bring about the preferential integration of protospacer into the leader proximal end.

© 2016 Indian Academy of Sciences, Bengaluru.