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 spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). Oligomerizes as an infinite helical thread in crystal structure; disrupting a potential salt bridge between Asp-35 and Arg-55 leads to altered elution from a sizing column. By immunoprecipitation Csa5 interacts weakly with a Cas5/Cas7 complex. There are 3 csa5, 3 cas5 and 2 cas7 genes in this organism; the immunoprecipitation studies may not distinguish between them.