Inducible CRISPR/Cas9 Allows for Multiplexed and Rapidly Segregated Single-Target Genome Editing in Synechocystis Sp. PCC 6803.

Establishing various synthetic biology tools is crucial for the development of cyanobacteria for biotechnology use, especially tools that allow for precise and markerless genome editing in a time-efficient manner. Here, we describe a riboswitch-inducible CRISPR/Cas9 system, contained on a single replicative vector, for the model cyanobacterium Synechocystis sp. PCC 6803. A theophylline-responsive riboswitch allowed tight control of Cas9 expression, which enabled reliable transformation of the CRISPR/Cas9 vector into Synechocystis . Induction of the CRISPR/Cas9 mediated various types of genomic edits, specifically deletions and insertions of varying size. The editing efficiency varied depending on the target and intended edit; smaller edits performed better, reaching, e.g., 100% for insertion of a FLAG-tag onto rbcL . Importantly, the single-vector CRISPR/Cas9 system mediated multiplexed editing of up to three targets in parallel in Synechocystis . All single-target and several double-target mutants were also fully segregated after the first round of induction. Lastly, a vector curing system based on the nickel-inducible expression of the toxic mazF (from Escherichia coli ) was added to the CRISPR/Cas9 vector. This inducible system allowed for curing of the vector in 25-75% of screened colonies, enabling edited mutants to become markerless.