Identification of collateral RNA cleavage mediated by Type VI CRISPR-Cas system from Leptotrichia shahiii

We investigated the in vivo and in vitro specificity of collateral RNA degradation mediated by LshCas13a protein encoded by Type VI CRISPR-Cas system derived from L. shahii. In our experimental system, we used Escherichia coli strain harboring Type VI spacer matching inducible transcript ("targeting" cells) or strain that does not encode spacers matching any E. coli transcripts ("nontargeting" cells). After the induction of the target transcription, cells were collected, total RNA samples were extracted from the cells and subjected to high throughput RNA sequencing with a specific protocol allowing to capture RNA degradation products. To investigate the influence of RNase toxins encoded in E. coli genome on the observed RNA degradation pattern, we repeated the described experiment on E. coli strain lacking ten known ribonuclease-encoding toxin-antitoxin loci. To investigate in vitro specificity of LshCas13a protein, we analyzed products of the degradation on total E. coli RNA by purified LshCas13a supplemented with crRNA and targeted or nontargeted transcripts. The obtained samples were processed as it was described above. All experiments were independently performed in triplicate. To detect transcript degradation sites, the obtained read pairs were filtered using trimmomatic tool and then mapped onto reference sequences using bowtie2. Next, for each nucleotide position of each strand of reference sequences the number of 5' ends of aligned fragments were counted producing corresponding tables. The differences between the numbers of mapped 5' ends in targeting and nontargeting samples were analyzed using edgeR package. Using this data, we identified target preferences for collateral RNA degradation by LshCas13a effectors. Overall design: The differences between targeting and nontargeting samples were analyzed. Each experiment was performed in triplicate, i. e. each analysis included six samples (3 targeting + 3 nontargeting).