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Promoters control the binding of RNA polymerase and transcription factors. Since the promoter region drives transcription of a target gene, it therefore determines the timing of gene expression and largely defines the amount of recombinant protein that will be produced. Many common promoters like T7, CMV, EF1A, and SV40, are always active and thus referred to as constitutive promoters. Others are only active under specific circumstances. In a previous post, we discussed inducible promoters, which can be switched from an OFF to an ON state, and how you might use these in your research. Today, we’ll look at repressible promoters, which can be switched from an ON to an OFF state, as well as repressible binary systems commonly used in Drosophila.

RNA-editing Cas13 enzymes have taken the CRISPR world by storm. Like RNA interference, these enzymes can knock down RNA without altering the genome, but Cas13s have higher on-target specificity. New work from Konermann et al. and Yan et al. describes new Cas13d enzymes that average only 2.8 kb in size and are easy to package in low-capacity vectors! These small, but mighty type VI-D enzymes are the latest tools in the transcriptome engineering toolbox.

CRISPR genome editing has made it easier to create knockout alleles in a variety of species, including the standard laboratory mouse. It’s also made targeted insertions relatively simple in C. elegans and bacteria. But CRISPRing typical mouse models, including creating Cre-dependent conditional alleles, has remained a challenge. Enter Easi-CRISPR: a method that harnesses the power of ssDNA donor molecules for homology directed repair. Using long ssDNA donors, the Gurumurthy and Ohtsuka groups have obtained an average knock-in efficiency of 30-60%. This is much more favorable than previous methods yielding 1-10% knock-in. Read on to learn how you can make CRISPR mouse model generation easi-er!

In order to bind DNA, Cas9 and other CRISPR enzymes require a short PAM sequence adjacent to the targeted sequence at the locus of interest. SpCas9’s 3’ NGG PAM occurs frequently in GC-rich genomes, but a PAM is not always available near the locus you’d like to modify. To tackle the PAM problem, researchers have engineered alternative Cas9s binding distinct PAM sequences. Now, Hu et al., working in David Liu’s lab, have gone one step further, using directed evolution to create xCas9, an enzyme recognizing a broad range of PAMs like NG, GAA, and GAT, but also displaying increased editing specificity. We’re excited to learn more about xCas9 - here’s what we know so far!

By mutating one of two Cas9 nuclease domains, researchers created the CRISPR nickase. Nickases create a single-strand rather than a double-strand break, and when used with two adjacent gRNAs, can lower the probability of off-target editing. But that’s not all! New research from IDT (Integrated DNA Technologies) has shown that a nickase approach can improve homology directed repair (HDR) rates, provided you follow some simple design rules described below.