Liu, Y., Wan, X. & Wang, B. Engineered CRISPRa enables programmable eukaryote-like gene activation in bacteria. Nat. Commun. 10, 3693 (2019).

In this publication, Liu and colleagues develop a CRISPRa system based on σ54-dependent promoters. These promoters are normally repressed by the σ54 factor, which creates a closed complex incapable of transcription at the promoter site. Gene expression is activated upon binding of bacterial Enhancer Binding Proteins (bEBPS) to an upstream activating sequence (UAS).
The group modified the CRISPR dCas9 sgRNA and designed protein linkers between the sgRNA and these activators. Further, they characterized core promoter sequences, artificial UASs (LEBs) and linker regions between the UAS and promoters, along with testing a design with dxCas9, which is less limited in the sequences it can target.
Here, 'Low' and 'High' mean basal levels of gene expression, and levels upon induction of the gRNA, given the other system components are expressed constitutively.