<p dir="ltr">The CRISPR-Cas systems, originally evolved as bacterial and archaeal adaptive immune systems against viral infections, have been ingeniously repurposed for genome editing. The ongoing evolutionary competition between bacteria and phages has given rise to the diversification of CRISPR-Cas systems, which can be broadly classified into two classes and six types. Among these, the versatile CRISPR type V family stands out as a promising source for discovering new CRISPR-Cas effectors to expand the genome editing toolbox. However, before proceeding to genome editing applications, it is imperative to get a comprehensive understanding of the mechanisms underlying how Cas effectors function as programmable RNA-guided nucleases. Structural studies play a pivotal role in elucidating these mechanisms, providing a clear picture of processes such as DNA recognition and cleavage.</p><p><br></p><p dir="ltr">In the first part of this thesis, we embarked on determining the cryo-EM structures of an extraordinarily small type V-F CRISPR-Cas effector, Cas12f. Our findings unveiled that Cas12f functions as an asymmetric dimer. Through structural analysis and mutagenesis experiments, we elucidated the mechanisms of PAM recognition and substrate cleavage by Cas12f. Furthermore, we provided insights into the activation mechanism of Cas12f by monitoring its conformational changes before and after the crRNA-target DNA heteroduplex formation. Our results contribute to our understanding of the type V Cas effector nucleases and hold promise for possible applications of genome editing.</p><p><br></p><p dir="ltr">In the second part, we focused on study of CRISPR-associated transposons (CASTs). Specifically, we delved into Cas12k, a component of the type V-K CRISPR-Cas system, which is a naturally inactivated nuclease but is interestingly associated with transposons and is capable for guiding transposition. We determined the structure of Cas12k in complex with the guide RNA and target DNA. Our studies revealed target site recognition mechanism and the structural features of Cas12k critical for downstream CIRPSR-guided DNA transposition.</p><p><br></p><p dir="ltr">Lastly, we directed our attention towards the ancestor of CRISPR type V systems, TnpB, which serves as a minimal programmable RNA-guided DNA nuclease originating from the IS200/IS605-like transposon family. To reveal the molecular mechanisms of substrate recognition and cleavage, multiple approaches including artificial dimers was introduced to obtained the cryo-EM structure of <i>Isdra2</i> TnpB-gRNA-target DNA ternary complex. Furthermore, our exploration extended to the investigation of newly emerged TnpB variants. Among these variants, one was identified as a naturally occurring transcription repressor. We attained the cryo-EM structure of this variant at 3.12 Å and currently working on understanding its mechanism.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/26357608 |
| Date | 25 July 2024 |
| Creators | Renjian Xiao (8992832) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/_b_Structural_and_functional_studies_of_type_v_crispr_b_b_-_b_b_cas_effectors_b_/26357608 |
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