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 (spacer sequences) and target invading nucleic acids. Unlike many single-component effectors, this CRISPR-Cas system targets RNA (PubMed:27669025). CRISPR clusters are transcribed from pre-CRISPR RNA (crRNA) and processed into crRNA by this protein (PubMed:27669025, PubMed:28475872, PubMed:28757251). pre-crRNA processing yields a 5'-OH and probably a 2',3'-cyclic phosphate (PubMed:27669025). Also cleaves pre-crRNA from several other type VI-A CRISPR systems (PubMed:28475872). Cleaves linear target ssRNA in a crRNA-dependent fashion, preferentially before U residues (PubMed:27669025, PubMed:28475872). Cleavage of target ssRNA is about 80-fold faster than pre-crRNA processing and uses a different active site (PubMed:27669025). Binding a viable target RNA target activates this protein for non-specific RNA degradation in vitro (called collateral RNA degradation) (PubMed:27669025, PubMed:28475872, PubMed:28757251). Activation occurs with 10 fM target RNA (PubMed:28475872). crRNA maturation is not essential for activation of RNA degradation, but lack of mature crRNA (due to mutagenesis) decreases activation levels (PubMed:28475872). This system has a 3' protospacer flanking site in the target RNA (PFS), which is C and unavailable to base pair with crRNA (PFS is equivalent to PAM, the protospacer adjacent motif) (PubMed:28757251). Pre-crRNA processing is metal independent, while crRNA-guided target RNA cleavage is dependent on divalent metal (i.e. inhibited by EDTA) (PubMed:27669025). Target RNA acts as an activator for non-specific ssRNA cleavage; the target RNA and complementary crRNA must both be at least 20 nucleotides long to activate the HEPN-like catalytic pocket for RNase activity (PubMed:28757251). Crystals show the 3'-end of target RNA interacting with an adjacent protein molecule, and mutagenesis of those amino acid residues decreases target RNA cleavage, but it is not clear if this is physiological (PubMed:28757251). The X-ray structure in complex with crRNA and target RNA shows a crRNA-recognition lobe (REC, residues 1-360) which anchors the crRNA repeat and a nuclease lobe (NUC, residues 361-1159) which binds the spacer crRNA-target RNA duplex and cleaves target RNA (PubMed:28757251). The target ssRNase active sites are within the 2 HEPN-like folds, which interact in vivo to form the functional active site (PubMed:26593719, PubMed:28757251). The N-terminal region and HEPN-like fold 2 play a role in pre-crRNA processing (PubMed:28475872). Binding of target RNA to the Cas13a-crRNA complex induces conformational changes that include bringing the 2 HEPN-like domains together to form the target cleavage active site, widening the channel that binds the crRNA-target RNA duplex, facilitating contact between the NUC lobe and the RNA duplex, and altering the conformation of the crRNA (PubMed:28757251). Can be used to detect low levels of cellular transcripts, which might be useful to create sequence-specific RNA-targeting tools (PubMed:27669025). Part of a type VI-A uridine-prefering CRISPR-Cas system. Belongs to the CRISPR-associated endoribonuclease Cas13a family.