Use of an intergenic region in Pseudomonas syringae pv. syringae B728a for site-directed genomic marking of bacterial strains for field experiments.

To construct differentially-marked derivatives of our model wild-type strain, Pseudomonas syringae pv. syringae B728a (a causal agent of bacterial brown spot disease in snap bean plants), for field experiments, we selected a site in the gacS-cysM intergenic region for site-directed insertion of antibiotic resistance marker cassettes. In each of three field experiments, population sizes of the site-directed chromosomally marked B728a derivatives in association with snap bean plants were not significantly different from that of the wild-type strain. Inserts of up to 7 kb of DNA in the intergenic region did not measurably affect fitness of B728a in the field. The site is useful for site-directed genomic insertions of single copies of genes of interest.