Why has spCas9 experienced such widespread adoption? My understanding is that there are several type II systems with comparable fidelity and efficacy, some like mnCas9 and saCas9 have the added advantage of being smaller. Is it simply historical reasons or is there a deeper driver that I am missing here? Thanks and much appreciated, Jon.
One of the main reasons is the relatively abundant PAM (protospacer adjacent motif) sequence of spCas9: NGG. This sequence is highly abundant throughout the genome, so this will give you a large amount of options for the sequence to target your sgRNA against. For instance, ST1 and NM require PAMs NNAGAAW and NNNNGATT respectively. These are far less abundant, and thus give far less targets in the genome to work with. SaCas9 is indeed a lot smaller than spCas9, and this can be advantageous. However, its optimal PAM is NNGRRT.
Ofcourse, another part of the reason is also for historical reasons. There may be Cas9 species with other 3nt PAM sites. However, since the first one that was discovered was SpCas9, most systems have made adaptations of this Cas9 protein so sgRNAs (and sgRNA design tools) can be used for multiple systems (including dCas9 activation / inhibition).
One of the main reasons is the relatively abundant PAM (protospacer adjacent motif) sequence of spCas9: NGG. This sequence is highly abundant throughout the genome, so this will give you a large amount of options for the sequence to target your sgRNA against. For instance, ST1 and NM require PAMs NNAGAAW and NNNNGATT respectively. These are far less abundant, and thus give far less targets in the genome to work with. SaCas9 is indeed a lot smaller than spCas9, and this can be advantageous. However, its optimal PAM is NNGRRT.
Ofcourse, another part of the reason is also for historical reasons. There may be Cas9 species with other 3nt PAM sites. However, since the first one that was discovered was SpCas9, most systems have made adaptations of this Cas9 protein so sgRNAs (and sgRNA design tools) can be used for multiple systems (including dCas9 activation / inhibition).
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