Use of a counterselectable transposon to create markerless knockouts from a 18,432-clone ordered M. bovis BCG mutant resource

Mutant resources are essential to improve our understanding of the biology of slow-growing mycobacteria, which include the causative agents of tuberculosis in various species, including humans. The generation of deletion mutants in slow-growing mycobacteria in a gene-by-gene approach in order to make genome-wide ordered mutant resources is still a laborious and costly approach; despite the recent development of improved methods. On the other hand, transposon mutagenesis in combination with Cartesian Pooling-Coordinate Sequencing allows the creation of large archived transposon insertion libraries. However, such mutants contain selection marker genes with a risk of polar gene effects, which is undesired both for research and for use of these mutants as live attenuated vaccines. In this paper, a derivative of the Himar1 transposon is described, which allows the generation of clean, markerless knockouts from archived transposon libraries. By incorporating sites for FlpE/-mediated recombination and sites for ISceIM-based transposon removal, we enable two thoroughly experimentally validated possibilities to create unmarked mutants from such marked transposon mutants. The approach is highly efficient but leaves an scar in the genome, whereas the mediated approach can create mutants without any heterologous DNA in the genome. The combined use of CP-CSeq and this optimized transposon was applied in the BCG Danish 1331 vaccine strain (WHO reference 07/270), creating the largest ordered, characterized resource of mutants in a member of the complex (18,432 clones, mutating 83% of the non-essential homologues), from which clean knockouts can be generated.