Cross-cleavage activity of Cas6b in crRNA processing of two different CRISPR-Cas systems in Methanosarcina mazei Gö1.

The clustered regularly interspaced short palindromic repeat (CRISPR) system is a prokaryotic adaptive defense system against foreign nucleic acids. In the methanoarchaeon Methanosarcina mazei Gö1, two types of CRISPR-Cas systems are present (type I-B and type III-C). Both loci encode a Cas6 endonuclease, Cas6b-IB and Cas6b-IIIC, typically responsible for maturation of functional short CRISPR RNAs (crRNAs). To evaluate potential cross cleavage activity, we biochemically characterized both Cas6b proteins regarding their crRNA binding behavior and their ability to process pre-crRNA from the respective CRISPR array in vivo. Maturation of crRNA was studied in the respective single deletion mutants by northern blot and RNA-Seq analysis demonstrating that in vivo primarily Cas6b-IB is responsible for crRNA processing of both CRISPR arrays. Tentative protein level evidence for the translation of both Cas6b proteins under standard growth conditions was detected, arguing for different activities or a potential non-redundant role of Cas6b-IIIC within the cell. Conservation of both Cas6 endonucleases was observed in several other M. mazei isolates, though a wide variety was displayed. In general, repeat and leader sequence conservation revealed a close correlation in the M. mazei strains. The repeat sequences from both CRISPR arrays from M. mazei Gö1 contain the same sequence motif with differences only in two nucleotides. These data stand in contrast to all other analyzed M. mazei isolates, which have at least one additional CRISPR array with repeats belonging to another sequence motif. This conforms to the finding that Cas6b-IB is the crucial and functional endonuclease in M. mazei Gö1. Abbreviations: sRNA: small RNA; crRNA: CRISPR RNA; pre-crRNAs: Precursor CRISPR RNA; CRISPR: clustered regularly interspaced short palindromic repeats; Cas: CRISPR associated; nt: nucleotide; RNP: ribonucleoprotein; RBS: ribosome binding site.