Archaea, a group of microorganisms distinct from bacteria and
eukaryotes, are equipped with an adaptive immune system called
the CRISPR system, which relies on an RNA interference mechanism
to combat invading viruses and plasmids. Using a genome
sequence analysis approach, the four components of archaeal
genomic CRISPR loci were analysed, namely, repeats, spacers,
leaders and cas genes. Based on analysis of spacer sequences it
was predicted that the immune system combats viruses and plasmids
by targeting their DNA. Furthermore, analysis of repeats,
leaders and cas genes revealed that CRISPR systems exist as distinct
families which have key differences between themselves.
Closely related organisms were seen harbouring different CRISPR
systems, while some distantly related species carried similar
systems, indicating frequent horizontal exchange. Moreover, it
was found that cas genes of Type I CRISPR systems could be divided
into functionally independent modules which occasionally
exchange to form new combinations of Type I systems. Furthermore,
Type III systems were found to be genomically associated
with various combinations of accessory genes which may play a
role in functionally extending the activity of the Type III interference
complexes. This dynamic nature of the CRISPR immune
systems may be a prerequisite for their continued efficacy against
the ever changing threats they protect their hosts from.