<P> Type III systems also use Cas6, however their repeats do not produce stem - loops . Cleavage instead occurs by the longer transcript wrapping around the Cas6 to allow cleavage just upstream of the repeat sequence . </P> <P> Type II systems lack the Cas6 gene and instead utilize RNaseIII for cleavage . Functional type II systems encode an extra small RNA that is complementary to the repeat sequence, known as a trans - activating crRNA (tracrRNA). Transcription of the tracrRNA and the primary CRISPR transcript results in base pairing and the formation of dsRNA at the repeat sequence, which is subsequently targeted by RNaseIII to produce crRNAs . Unlike the other two systems the crRNA does not contain the full spacer, which is instead truncated at one end . </P> <P> CrRNAs associate with Cas proteins to form ribonucleotide complexes that recognize foreign nucleic acids . CrRNAs show no preference between the coding and non-coding strands, which is indicative of an RNA - guided DNA - targeting system . The type I-E complex (commonly referred to as Cascade) requires five Cas proteins bound to a single crRNA . </P> <P> During the interference stage in type I systems the PAM sequence is recognized on the crRNA - complementary strand and is required along with crRNA annealing . In type I systems correct base pairing between the crRNA and the protospacer signals a conformational change in Cascade that recruits Cas3 for DNA degradation . </P>

What are the two main molecular components of the crispr system