CRISPR interference: RNA-directed adaptive immunity in bacteria and archaea

Top Cited Papers
Publisher Website
Google Scholar
Abstract
Recently, arrays of clustered, regularly interspaced short palindromic repeats (CRISPRs) have been implicated in a novel genetic interference pathway that limits phage infection and plasmid conjugation. CRISPR loci keep a record of past infections to provide bacteria and archaea with a 'genetic memory' that directs the rejection of invader DNA molecules; therefore these loci constitute an adaptive immune system. CRISPRs (which are approximately 25–50 nucleotides long) are separated by similarly short sequences called spacers that match bacteriophage or plasmid sequences and specify the targets of interference. CRISPR-associated (cas) genes, a set of conserved genes that are associated with these loci, are usually present on one or the other side of the array. CRISPR loci are transcribed as a long precursor that is processed by Cas proteins within the repeat sequences to generate small CRISPR RNAs (crRNAs). The crRNAs serve as guides for target recognition during CRISPR interference. crRNA–Cas ribonucleoprotein complexes seem to generally target invading DNA sequences during interference but may also target RNAs in some species. Upon bacteriophage challenge of a CRISPR-containing bacterial population, mutants resistant to the infection arise through the incorporation of additional spacer sequences derived from the challenging phage. This allows the bacteria to evolve rapidly and adapt to the viruses in the environment. Bacteriophages and plasmids can mobilize foreign genetic material between cells, a process known as horizontal gene transfer (HGT), which is a fundamental source of genetic variability for the evolution of bacteria and archaea. CRISPR interference prevents phage infection and plasmid conjugation and therefore constitutes a natural barrier to HGT. In addition, bacteriophages constantly mutate to evade CRISPR defence. Therefore, CRISPR interference has an important role in the evolution of microbial communities.