Int.J.Curr.Microbiol.App.Sci (2015) 4(6): 187-200 187 Review Article CRISPR Systems: RNA-Guided Defence Mechanisms in Bacteria and Archaea Manish Dhawan 1 *, Manish Sharma 2 and Rajveer Singh Grewal 3 1 Department of Microbiology, Punjab Agricultural University, Ludhiana, India 2 Department of Microbiology, Guru Nanak Dev University, Amritsar, India 3 School of Agricultural Biotechnology, Punjab Agricultural University, Ludhiana, India *Corresponding author ABSTRACT Introduction Viruses are responsible for 4 to 50% destruction of bacterial and archaeal communities (Breitbart and Rohwer, 2005; Rohwer and Thurber, 2009) and the selective pressures imposed by these rapidly evolving viruses (parasites) has driven the diversification of microbial defence systems. Previously our understanding of antiviral immune systems in bacteria has focused on encounter blocks, adsorption resistance, penetration blocks, abortive infection, restriction modification, phage growth limitation systems (Hyman and Abedon, 2010; Labrie et al., 2010). Now, our understanding has been expanded to include the CRISPR systems of Clustered Regularly Interspaced Short Palindromic Repeats. These CRISPR systems are essential International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 4 Number 6 (2015) pp. 187-200 http://www.ijcmas.com Bioinformatics, genetics and biochemical studies have revealed that many prokaryotes use an RNA-based adaptive immune system to target and destroy genetic parasites. Prokaryotic Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) systems provide a sophisticated adaptive immune system that offers protection against foreign nucleic acid. CRISPR-Cas system consists of CRISPR loci contains array of identical repeats interspaced with spacers carrying unique sequences and CRISPR-associated (Cas) proteins exhibit helicases, nucleases, polymerases and RNA-binding functions. In CRISPR-based immune systems, short sequence tags from invading genetic elements (Protospacers) are actively incorporated into the host’s CRISPR locus to be transcribed and processed into library of short CRISPR-derived RNAs (crRNAs). These crRNAs packaged into a large surveillance complex which guides destruction of foreign genetic material. CRISPR-Cas systems have been grouped into three types (Types I–III), along with subtypes (e.g., Type I-E), on the basis of the Cas genes they possess and their mode of action. CRISPR system proves useful in genome editing, development of industrially important phage resistant strains, reduction of antibiotic resistance (horizontal gene flow). Discovery of fundamental mechanisms of CRISPR systems raised many new questions such as how and when target sequences are identified during a phage infection. Keywords Cas proteins, CRISPR, crRNAs, Repeats and Spacers