The nuts and bolts of the Haloferax CRISPR-Cas system I-B.

Invading genetic elements pose a constant threat to prokaryotic survival, requiring an effective defence. Eleven years ago, the arsenal of known defence mechanisms was expanded by the discovery of the CRISPR-Cas system. Although CRISPR-Cas is present in the majority of archaea, research often focuses on bacterial models.

A complex of Cas proteins 5, 6, and 7 is required for the biogenesis and stability of clustered regularly interspaced short palindromic repeats (crispr)-derived rnas (crrnas) in Haloferax volcanii.

The clustered regularly interspaced short palindromic repeats/CRISPR-associated (CRISPR-Cas) system is a prokaryotic defense mechanism against foreign genetic elements. A plethora of CRISPR-Cas versions exist, with more than 40 different Cas protein families and several different molecular approaches to fight the invading DNA. One of the key players in the system is the CRISPR-derived RNA (crRNA), which directs the invader-degrading Cas protein complex to the invader.

Requirements for a successful defence reaction by the CRISPR-Cas subtype I-B system.

Uptake of foreign mobile genetic elements is often detrimental and can result in cell death. For protection against invasion, prokaryotes have developed several defence mechanisms, which take effect at all stages of infection; an example is the recently discovered CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated) immune system. This defence system directly degrades invading genetic material and is present in almost all archaea and many bacteria.

The immune system of halophilic archaea.

Prokaryotes have developed several strategies to defend themselves against foreign genetic elements. One of those defense mechanisms is the recently identified CRISPR/Cas system, which is used by approximately half of all bacterial and almost all archaeal organisms. The CRISPR/Cas system differs from the other defense strategies because it is adaptive, hereditary and it recognizes the invader by a sequence specific mechanism.

The ring of confidence: a haloarchaeal CRISPR/Cas system.

To survive the constant invasions by foreign genetic elements, prokaryotes have evolved various defensive systems. Almost all sequenced archaea, and half of the analysed bacteria use the CRISPR (clustered regularly interspaced short palindromic repeats)/Cas (CRISPR-associated) system, a recently identified prokaryotic immune system that can fend off invading elements in a sequence-specific manner. Few archaeal CRISPR/Cas systems have been analysed so far, and the molecular details of many of the steps involved in adaptation and defence are yet to be understood.

An archaeal immune system can detect multiple protospacer adjacent motifs (PAMs) to target invader DNA.

The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system provides adaptive and heritable immunity against foreign genetic elements in most archaea and many bacteria. Although this system is widespread and diverse with many subtypes, only a few species have been investigated to elucidate the precise mechanisms for the defense of viruses or plasmids. Approximately 90% of all sequenced archaea encode CRISPR/Cas systems, but their molecular details have so far only been examined in three archaeal species: Sulfolobus solfataricus, Sulfolobus islandicus, and Pyrococcus furiosus.

Small RNAs for defence and regulation in archaea.

Non-coding RNAs are key players in many cellular processes within organisms from all three domains of life. The range and diversity of small RNA functions beyond their involvement in translation and RNA processing was first recognized for eukaryotes and bacteria. Since then, small RNAs were also found to be abundant in archaea.