Abstract: Engineered Streptococcus canis Cas9 (ScCas9) variants include an ScCas9 protein with its PID being the PID amino acid composition of Streptococcus pyogenes Cas9 (SpCas9)-NG, an ScCas9 protein having a threonine-to-lysine substitution mutation at position 1227 in its amino acid sequence (Sc+), and an ScCas9 protein having a threonine-to-lysine substitution mutation at position 1227 and a substitution of residues ADKKLRKRSGKLATE in position 365-379 in the ScCas9 open reading frame (Sc++). Also included are CRISPR-associated DNA endonucleases with a PAM specificity of 5?-NG-3? or 5?-NNG-3? and a method of altering expression of a gene product by utilizing the engineered ScCas9 variants.

Abstract: A Streptococcus canis Cas9 (ScCas9) ortholog and its engineered variants, possessing novel PAM specificity, is an addition to the family of CRISPR-Cas9 systems. ScCas9 endonuclease is used in complex with guide RNA, consisting of identical non-target-specific sequence to that of the guide RNA SpCas9, for specific recognition and activity on a DNA target immediately upstream of either an “NNGT” or “NNNGT” PAM sequence. A novel DNA-interacting loop domain within ScCas9, and other Cas9 orthologs, such as those from Streptococcus gordonii and Streptococcus angionosis facilitates a divergent PAM sequence from the “NGG” PAM of SpCas9.

Abstract: A chimeric DNA:RNA guide for very high accuracy Cas9 genome editing employs nucleotide-type substitutions in nucleic acid-guided endonucleases for enhanced specificity. The CRISPR-Cas9 gene editing system is manipulated to generate chimeric DNA:RNA guide strands to minimize the off-target cleavage events of the S. pyogenes Cas9 endonuclease. A DNA:RNA chimeric guide strand is sufficient to guide Cas9 to a specified target sequence for indel formation and minimize off-target cleavage events due to the specificity conferred by DNA-DNA interactions. Use of chimeric mismatch-evading lowered-thermostability guides (“melt-guides”) demonstrate that nucleotide-type substitutions in the spacer can reduce cleavage of sequences mismatched by as few as a single base pair. The chimeric mismatch-evading lowered-thermostability guides replace most gRNA spacer positions with DNA bases to suppress mismatched targets under Cas9's catalytic threshold.