Abira Chaudhuri, Koushik Halder & Asis Datta

Theoretical and Applied Genetics; February 2022; vol. 135: 367–387

Abstract

Remarkable diversity in the domain of genome loci architecture, structure of effector complex, array of protein composition, mechanisms of adaptation along with difference in pre-crRNA processing and interference have led to a vast scope of detailed classification in bacterial and archaeal CRISPR/Cas systems, their intrinsic weapon of adaptive immunity. Two classes: Class 1 and Class 2, several types and subtypes have been identified so far. While the evolution of the effector complexes of Class 2 is assigned solely to mobile genetic elements, the origin of Class 1 effector molecules is still in a haze. Majority of the types target DNA except type VI, which have been found to target RNA exclusively. Cas9, the single effector protein, has been the primary focus of CRISPR-mediated genome editing revolution and is an integral part of Class 2 (type II) system. The present review focuses on the different CRISPR types in depth and the application of CRISPR/Cas9 for epigenome modification, targeted base editing and improving traits such as abiotic and biotic stress tolerance, yield and nutritional aspects of tomato breeding.

See: https://link.springer.com/article/10.1007/s00122-021-03984-y

Figure 1: Type IIA CRISPR/Cas system in Streptococcus pyogenes. a CRISPR locus in genome: CRISPR locus consists of CRISPR repeats and spacer array along with tracrRNA and Cas operon. New spacers from the invading organism are incorporated into this CRISPR array (adaptation). b crRNA:tracrRNA co-maturation and complex formation with Cas9: pre-crRNA becomes matured crRNA with the help of tracrRNA along with Csn1 and Rnase III. Matured crRNA with Cas9 gets incorporated into ribonucleoprotein complexes which start scanning for nucleic acids complimentary to the sequence coded by crRNA (maturartion). c RNA-guided Cas9-mediated cleavage of targeted DNA: Upon recognition, complementary foreign sequences are cleaved by this Cas protein complex (Interference)