CRISPR (clustered regularly interspaced short palindromic repeat), is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain sequences complementary to antecedent mobile elements (spacers) and target invading nucleic acids. Unlike many single-component effectors, this CRISPR-Cas system targets RNA (PubMed:27256883, PubMed:28086085). CRISPR clusters are transcribed from pre-CRISPR RNA (crRNA) and processed into crRNA (optimally 28 nucleotides in this system) by this protein (PubMed:27256883, PubMed:27669025, PubMed:28475872, PubMed:28086085). This protein processes pre-crRNA at a 'non-typical' site 1 nucleotide upstream of the pre-crRNA stem-loop; it cleaves pre-crRNA from L.buccalis and L.wadei in a similar fashion, whereas the enzymes from the latter 2 bacteria cleave their own pre-crRNA 3 nt further upstream (PubMed:28475872). When the appropriate target sequences are cloned into the CRISPR array, confers immunity to ssRNA(+) enterobacteria phage MS2 (PubMed:27256883). Cleaves linear target ssRNA in a crRNA-dependent fashion, preferentially before U residues; has no activity on partially dsRNA, ssDNA or dsDNA (PubMed:27256883). RNA secondary structure surrounding the target influence the cleavage site and efficiency; unlike other CRISPR-Cas effectors Cas13a cleaves outside of the crRNA binding site (PubMed:27256883). In the presence of a viable RNA target other RNAs are also degraded (called collateral RNA degradation), suggesting this type of CRISPR-Cas might also prevent viral spread by inducing programmed cell death or dormancy (PubMed:27256883). This system has a 3' protospacer flanking site (PFS), it does not cleave when the 3' PFS is G (PFS is equivalent to PAM, the protospacer adjacent motif) (PubMed:27256883). Mutations of its active site residues results in an RNA-programmed RNA-binding protein (PubMed:27256883). ssRNA target cleavage requires Mg(2+), weak target cleavage is seen with Mn(2+) and Ca(2+). RNase activity on target is decreased by EDTA (PubMed:27256883). Target RNA acts as an activator for non-specific ssRNA degradation (PubMed:27256883). Monomer (PubMed:27256883). The target ssRNase active sites are within the 2 HEPN-like folds which interact in vivo (PubMed:26593719, PubMed:27256883, PubMed:28086085). The X-ray structure in complex with crRNA shows a crRNA-recognition lobe (REC, residues 1-498) which makes most of the contacts with crRNA and is essential for pre-crRNA cleavage, and a nuclease lobe (NUC, residues 499-1389) which also makes a few contacts with the crRNA repeat and is responsible for target ssRNA cleavage (PubMed:28086085). Binding of crRNA induces conformational changes that probably stabilize crRNA-binding and facilitate target ssRNA recognition (PubMed:28086085). Mutagenesis of the crRNA and this protein show the stem loop and 3'-direct repeat of the crRNA are essential for target ssRNase while the 5'-end of the crRNA and its bulge-containing stem are essential for pre-crRNA cleavage (PubMed:28086085). Can be reprogrammed to target specific mRNAs in an E.coli system, which might be useful to create specific RNA-targeting tools (PubMed:27256883). Part of a type VI-A uridine-prefering CRISPR-Cas system. Belongs to the CRISPR-associated endoribonuclease Cas13a family.