Difference between revisions of "Part:BBa K3738026"

Latest revision as of 22:57, 21 October 2021

Lbu-Cas13a with an N-terminal 6xHistidine Tag and C-Terminal Anionic Tag and crRNA

Cas13a is an enzyme originating from Leptotrichia buccalis (Lbu), which functions to cleave single-stranded RNAs (ssRNAs); particularly mRNAs. This function is achieved following protein-RNA complex formation with CRISPR RNA (crRNA) via crRNA backbone contacts with residues from the Helical-2, HEPN1, and Linker domains of Cas13a. The crRNA contains a spacer region coding for a direct repeat stem loop, as well as a region complementary to target ssRNAs. Once the enzyme complex interacts with a target ssRNA, a structural conformation change occurs within the domains of the protein that permits active site formation for non-discriminate ssRNA cleavage (O’Connel et al., 2019).

Lbu-crRNA (Lbu signifying Leptotrichia buccalis, the source of the Cas13a protein and crRNA) is an RNA structure containing a direct-repeat stem loop, a recognition element for binding with the Lbu-Cas13a protein. The crRNA also contains a downstream complementary region, designed to base-pair with a target ssRNA sequence. Complex formation of crRNA-Cas13a occurs, and when the target sequence is perfectly paired with the crRNA, activation of the enzyme occurs and subsequent non-discriminate cleavage of collateral ssRNAs (O’Connell., 2019).

This crRNA was designed to bind with an important protein in the synthesis of harmful toxins called microcystins produced by blue-green algae (cyanobacteria) blooms. McyH is a gene in the Mcy gene cluster of Microcystic Aeruginosa and codes for a transporter protein. The protein is comprised of two homodimers, each with a hydrophobic N-terminus domain and C-terminus containing an ATPase domain. Pearson et al., 2004 experimentally examined the impacts of impairing expression of this gene, and combined with bioinformatic data, found that McyH is likely a vital exporter of harmful microcystins and essential in their biosynthetic pathway. For this reason, we have designed crRNA to target the 5’-end of McyH in order to inhibit production of microcystins.

This composite part includes the IPTG-inducible T7 promoter (BBa_J64997), RBS BBa_B0034, Lbu-Cas13a with an N-terminal 6xHistidine Tag and C-Terminal Anionic Tag coding region (BBa_K3738020), and double terminator BBa_B0015, followed by a second T7 promoter (BBa_J64997), McyH complementary Lbu-crRNA (BBa_K3738025), and a second BBa_B0015 terminator. The part is codon-optimized for use in E. coli It is improved from the Lethbridge High School iGEM 2019's Parts BBa_K3001003, BBa_K3001000 and BBa_K3001002 by introducing the anionic MS2 phage-like-particle uptake anionic tag as well as the transcriptional and translational regulators for optimal overexpression and 6XHistidine tag required for nickel affinity chromatography purification.

Sequence and Features