Lack of Cas13a inhibition by anti-CRISPR proteins from Leptotrichia prophages.

CRISPR systems are prokaryotic adaptive immune systems that use RNA-guided Cas nucleases to recognize and destroy foreign genetic elements. To overcome CRISPR immunity, bacteriophages have evolved diverse families of anti-CRISPR proteins (Acrs). Recently, Lin et al. (2020) described the discovery and characterization of 7 Acr families (AcrVIA1-7) that inhibit type VI-A CRISPR systems. We detail several inconsistencies that question the results reported in the Lin et al. (2020) study. These include inaccurate bioinformatics analyses and bacterial strains that are impossible to construct. Published strains were provided by the authors, but MS2 bacteriophage plaque assays did not support the published results. We also independently tested the Acr sequences described in the original report, in E. coli and mammalian cells, but did not observe anti-Cas13a activity. Taken together, our data and analyses prompt us to question the claim that AcrVIA1-7 reported in Lin et al. are type VI anti-CRISPR proteins.