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 and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). A component of Cascade, which participates in CRISPR interference, the third stage of CRISPR immunity. Cascade binds both crRNA and in a sequence-specific manner negatively supercoiled dsDNA target. This leads to the formation of an R-loop in which the crRNA binds the target DNA, displacing the noncomplementary strand. Cas3 is recruited to Cascade, probably via interactions with CasA, nicks target DNA and then unwinds and cleaves the target, leading to DNA degradation and invader neutralization. CasA is not required for formation of Cascade, but probably enhances binding to and subsequent recognition of both target dsDNA and ssDNA. Part of the Cascade ribonucleoprotein complex, with stoichiometry CasA(1),CasB(2),CasC(6),CasD(1),CasE(1)-crRNA(1). Interacts directly with the 5' end of crRNA, CasB, CasD and CasE. Binding of target ssRNA or dsDNA causes a conformational change in the Cascade complex; CasA is required for high affinity target DNA binding. Interacts with Cas3 once Cascade has recognized target DNA. Repressed by H-NS, activated by LeuO. Activated by the BaeSR two-component regulatory system, possibly due to envelope stress. Part of the casABCDE-ygbT-ygbF operon. Loss of resistance to bacteriophage lambda infection, loss of plasmid silencing. Belongs to the CRISPR associated protein CasA/Cse1 family. Type I-E/Ecoli subfamily.