Spirulina Raw sequence reads. Arthrospira

Cas1 integrase is the key enzyme of the CRISPR-Cas adaptation module that mediates acquisition of spacers derived from foreign DNA into the CRISPR arrays. In a diverse set of bacteria, Cas1 is fused to a reverse transcriptase (RT) domain that is related to the RTs of Group II introns and enables acquisition of CRISPR spacers from RNA. To compare the sequence pools sampled by RT-Cas1-associated and RT-lacking CRISPR-Cas systems, we obtained a series of independent samples from a cyanobacterial species Arthrospira platensis, grown commercially in open-air ‘raceway’ ponds. Sequencing of the CRISPR spacer repertoire as well as the intracellular and extracellular DNA and RNA pools uncovered a highly diverse population of spacers. However, the principal source(s) of the spacers, particularly the hyper-variable spacer repertoire of the RT-associated arrays, remains enigmatic.