Their effector module is a single multidomain protein (Cas9 in type II). They have different effector modules, composed of multiple Cas proteins. (*) based on the identification of signature genes for CRISPR–Cas types and subtypes.ĬRISPR-Cas systems are thus classified into 2 classes, 6 types and 33 subtypes. We present here the classification of Kira Makarova & al. This precursor is then matured into small crRNA that play a role in the targeting and destruction of homologous foreign sequences.Īll cas genes (13 plus many variants) are not shared by all CRISPR–Cas systems, leading to an enormous diversity. This is further shown by their presence on megaplasmids.ĬRISPR loci are transcribed into a pre-crRNA from the leader acting as a promoter. Phylogenetic studies performed on the CAS protein suggest that CRISPRs are acquired by horizontal transfer. Their number varies from one type to another. Perform the three different functions of the immune system: adaptation, crRNA maturation and interference. Several genes called cas for CRISPR-associated are found in the vicinity of CRISPRs and viruses, plasmids or mobile genetic elements. The unique sequences or spacers correspond mostly to fragments of foreign DNA, ie. The spacers in the different CRISPRs are different. In a given strain several CRISPRs can be found with a single or different repeat sequences but only one of each kind is associated Together with a set of genes called cas for “CRISPR-associated”, they constitute an immune system.ĬRISPR-associated genes are genes found closely linked to the repetitive sequences. The CRISPR locus is generally flanked on one side by an AT-rich leader sequence of 100-350 bp,Īcting as a promoter for the pre-crRNA synthesis. Sometimes, at one end of the CRISPR, the repeat is not totally conserved, it is called degenerate repeat. They have been described in a wide range of prokaryotes, including the majority of Archae and many Eubacteria.Ī CRISPR is a succession of 23-47bp sequences called repeats separated by unique sequences of a similar length (spacers). That show characteristics of both tandem and interspaced repeats. ( CRISPRs) are specific structures found in many prokaryotic genomes Clustered Regularly Interspaced Short Palindromic Repeats
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