Selective and hyperactive uptake of foreign DNA by adaptive immune systems of an archaeon via two distinct mechanisms

Central to the disparate adaptive immune systems of archaea and bacteria are clustered regularly interspaced short palindromic repeats (CRISPR). The spacer regions derive from invading genetic elements and, via RNA intermediates and associated proteins, target and cleave nucleic acids of the invader. Here we demonstrate the hyperactive uptake of hundreds of unique spacers within CRISPR loci associated with type I and IIIB immune systems of a hyperthermophilic archaeon. Infection with an environmental virus mixture resulted in the exclusive uptake of protospacers from a co-infecting putative conjugative plasmid. Spacer uptake occurred by two distinct mechanisms in only one of two CRISPR loci subfamilies present. In two loci, insertions, often multiple, occurred adjacent to the leader while in a third locus single spacers were incorporated throughout the array. Protospacer DNAs were excised from the invading genetic element immediately after CCN motifs, on either strand, with the secondary cut apparently produced by a ruler mechanism. Over a 10-week period, there was a gradual decrease in the number of wild-type cells present in the culture but the virus and putative conjugative plasmid were still propagating. The results underline the complex dynamics of CRISPR-based immune systems within a population infected with genetic elements.