A team of Massachusetts General Hospital (MGH) investigators has shown that a method they developed to improve the usefulness and precision of the most common form of the gene-editing tools CRISPR-Cas9 RNA-guided nucleases can be applied to Cas9 enzymes from other bacterial sources. In an article published online on November 2, 2015 in Nature Biotechnology, the team reports evolving a variant of SaCas9 - the Cas9 enzyme from the Streptococcus aureus bacteria - that recognizes a broader range of nucleotide sequences, allowing targeting of genomic sites previously inaccessible to CRISPR-Cas9 technology. The article is titled “Broadening the Targeting Range of Staphylococcus aureus CRISPR-Cas9 by Modifying PAM Recognition.” "The development of Cas9 variants with a broader targeting range is particularly important for applications requiring precise targeting of genomic sequences," says Benjamin Kleinstiver, Ph.D., a research fellow in the MGH Molecular Pathology Unit and lead and co-corresponding author of the new Nature Biotechnology paper. "In addition, the coding sequence of SaCas9 is 23 percent smaller than that of SpCas9 - the version derived from Streptococcus pyogenes - a size difference that makes SaCas9 advantageous for potential therapeutic applications requiring delivery by viruses." CRISPR-Cas9 nucleases are comprised of a short RNA molecule, 20 nucleotides of which match the target DNA sequence, and a Cas9 bacterial enzyme that cuts the DNA in the desired location. Along with the match between the RNA and DNA sequences, Cas9 needs to recognize an adjacent nucleotide sequence called a protospacer adjacent motif (PAM). In a previous study reported earlier this year in Nature, the MGH team described a genetic system that enabled them to rapidly evolve SpCas9 to recognize different PAM sequences.
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