Broad-host-range mutagenesis with CRISPR-associated transposase

Abstract

Transposons have been instrumental tools in microbiology enabling random mutagenesis, with transposons like Tn5 and Mariner, and site-specific DNA integrations with Tn7. However, programmable targeting of transposons was impossible until CRISPR-associated transposase (CasTn) systems were described. Like other CRISPR-derived systems, CasTn can be programmed with a short DNA encoded sequence that is transcribed into a guide-RNA. Here we describe a broad-host-range CasTn system and demonstrate its function in bacteria from three classes of the Proteobacteria. The CasTn genes are expressed from a broad-host-range replicative plasmid, while the guide-RNA and transposon are provided on a high-copy pUC plasmid that is suicidal in most bacteria outside of E. coli. Using our CasTn system, single-gene disruptions were performed with on-target efficiencies approaching 100% in the Beta- and Gammaproteobacteria, Burkholderia thailandensis, and Pseudomonas putida, respectively. The results were more modest in the Alphaproteobacterium Agrobacterium fabrum, with a peak efficiency of 45%, though for routine single-gene disruptions, this efficiency is adequate. In B. thailandensis, the system allowed simultaneous co-integration of transposons at two different target sites. The CasTn system is also capable of high-efficiency large transposon insertion totaling over 11 kbp in P. putida. Given the iterative capabilities and large payload size, this system will be helpful for genome engineering experiments across several fields of research. SignificanceThe genetic modification of bacteria to disrupt native genes and integrate recombinant genes is necessary for basic and applied research. Traditional methods for targeted disruptions and insertions are often cumbersome and inefficient, limiting experiments' scale and throughput. This work developed a system for targeted transposon mutagenesis that is easy to use, iterative, and efficient. We demonstrate that the system functions across three different classes of the Proteobacteria in species widely used in research and biotechnology. Moreover, the framework of the system and accompanying plasmids that we developed will facilitate porting the system to other bacteria. Our system provides a fast and efficient protocol to genetically modify these bacteria by inserting desired genetic cargo into specific genomic targets.