Arbuscular (AM) mycorrhizal fungi form symbiotic associations with the roots of plants that lead to increased growth and health of
many plants. The agricultural application of mycorrhizas has remained difficult since AM fungi are obligate biotrophs that show only
very limited growth in the absence of a host. As a first step in the molecular and genetic characterization of these fungi we have
constructed genomic libraries of Glomus versiforme and Gigaspora margarita. We demonstrated that in addition to fungal genes these
libraries contain clones derived from the genome of endosymbiotic bacteria that are present in fungal spores. A genomic clone
encoding elongation factor 1 alpha (EF-1) was isolated from G. versiforme and to our knowledge this is the first time a low copy
number gene was cloned from an AM fungus. The EF-1 promoter is highly active in mycorrhizal roots and will be an important tool
for the expression of foreign genes in the fungus.