Versatile Vectors for Efficient Mutagenesis of Bradyrhizobium diazoefficiens and Other Alphaproteobacteria

Analysis of bacterial gene function commonly relies on gene disruption or replacement followed by phenotypic characterization of the resulting mutant strains. Deletion or replacement of targeted regions is commonly achieved via two homologous recombination (HR) events between the bacterial genome and a nonreplicating plasmid carrying DNA fragments flanking the region to be deleted. The counterselection of clones that have integrated the entire plasmid in their genome via a single HR event is crucial in this procedure. Various genetic tools and well-established protocols are available for this type of mutagenesis in model bacteria; however, these methods are not always efficiently applicable in less established systems. Here we describe the construction and application of versatile plasmid vectors pREDSIX and pTETSIX for marker replacement and markerless mutagenesis, respectively. Apart from an array of restriction sites optimized for cloning of GC-rich DNA fragments, the vector backbone contains a constitutively expressed gene for mCherry, enabling the rapid identification of clones originating from single or double HR events by fluorescence-assisted cell sorting (FACS). In parallel, we constructed a series of plasmids from which gene cassettes providing resistance against gentamicin, kanamycin, hygromycin B, streptomycin and spectinomycin, or tetracycline were excised for use with pREDSIX-based marker replacement mutagenesis. In proof-of-concept mutagenesis experiments, we demonstrated the potential for the use of the developed tools for gene deletion mutagenesis in the nitrogen-fixing soybean symbiont Bradyrhizobium diazoefficiens(formerly Bradyrhizobium japonicum) and three additional members of the alphaproteobacteria.

Mutation and phenotypic analysis are essential to the study of gene function. Efficient mutagenesis protocols and tools are available for many bacterial species, including various model organisms; however, genetic analysis of less-well-characterized organisms is often impaired by the lack of efficient methods. Here we describe a set of novel genetic tools for facilitated mutagenesis of the nitrogen-fixing soybean symbiont Bradyrhizobium diazoefficiens and related alphaproteobacteria. We demonstrated their usefulness by generating several mutant strains lacking defined genes. Isolation of both antibiotic resistance gene-containing and markerless deletion mutants is greatly facilitated because undesired clones which contain the entire mutagenic plasmid integrated in the genome can be identified on the basis of their fluorescent phenotype derived from them Cherrygene carried by the vector backbone. The possibility to generate markerless mutants assists with the isolation of strains carrying multiple deletions, which can be crucial while studying functionally redundant genes.