A Burkholderia Strain Living Inside the Arbuscular Mycorrhizal Fungus Gigaspora margarita Possesses the vacB Gene,Which Is Involved in Host Cell Colonization by Bacteria

The arbuscular mycorrhizal (AM) fungus Gigaspora margarita harbors a resident population of endosymbiontic Burkholderia in its cytoplasm. Nothing is known about the acquisition of such bacteria and about the molecular bases which allow colonization of the fungus. We wondered whether the intracellular Burkholderia strain possesses genetic determinants involved in colonization of a eukaryotic cell. Using degenerated oligonucleotide primers for vacB, a gene involved in host cell colonization by pathogenic bacteria, an 842 bp DNA fragment was cloned, sequenced, and identified as a part of the vacB gene in Burkholderia sp. The insert was used as a probe to screen a fungal library that, because of the presence of intracellular Burkholderia cells, was also representative of the bacterial genome. The complete nucleotide sequence of vacB and flanking genes was determined. The bacterial origin of this genomic region was established by PCR, using specific vacB primers on DNA from Gigasporaceae that did or did not contain cytoplasmic Burkholderia, as well as on DNA from other bacteria, including free-living Burkholderia. We hypothesize that the vacB gene is part of a new genetic region acquired by a rhizospheric Burkholderia strain, which became able to establish a symbiotic interaction with the AM fungus G. margarita.