Nature and intensity of selection pressure on CRISPR-associated genes

The recently discovered CRISPR-Cas adaptive immune system is present in almost all archaea and many bacteria. It consists of cassettes of CRISPR repeats that incorporate spacers homologous to fragments of viral or plasmid genomes that are employed as guide RNAs in the immune response, along with numerous CRISPR-associated (cas) genes that encode proteins possessing diverse, only partially characterized activities required for the action of the system. Here, we investigate the evolution of the cas genes and show that they evolve under purifying selection that is typically much weaker than the median strength of purifying selection affecting genes in the respective genomes. The exceptions are the cas1 and cas2 genes that typically evolve at levels of purifying selection close to the genomic median. Thus, although these genes are implicated in the acquisition of spacers from alien genomes, they do not appear to be directly involved in an arms race between bacterial and archaeal hosts and infectious agents. These genes might possess functions distinct from and additional to their role in the CRISPR-Cas-mediated immune response. Taken together with evidence of the frequent horizontal transfer of cas genes reported previously and with the wide-spread microscale recombination within these genes detected in this work, these findings reveal the highly dynamic evolution of cas genes. This conclusion is in line with the involvement of CRISPR-Cas in antiviral immunity that is likely to entail a coevolutionary arms race with rapidly evolving viruses. However, we failed to detect evidence of strong positive selection in any of the cas genes.