CRISPR (clustered regularly interspaced short palindromic repeats) and piRNA cluster (Piwi-interacting Ribo nucleic acid) are two different defensive mechanisms used by bacteria to destroy ‘invasive’ nucleic acids like transposons and viruses.
CRISPR and piRNA produce long RNAs (similar to antisense RNA). These RNAs interact with and guide bacterial protein complexes to degrade complementary RNAs of transposons or viruses.
When a transposon or a virus inserts in the piRNA or a CRISPR locus itself, it leads to a permanent, genetic change in these loci. Thus, the inserts evade regulatory mechanisms of bacteria by disrupting the nucleic acid sequences of defensive piRNAor CRISPR.
While a virus leaves part of its genome into the said loci, a transposon may insert its entire sequence. This insertion either inactivates the loci, or may lead to new functions depending on the position of insertion of the invading sequences.