Expression of β-glucuronidase (GUS)-encoding reporter gene uidA inserted within SMc00911. S. meliloti within alfalfa root nodules (B–F) express GUS inserted in SMc00911 throughout the nodule. Panel A shows an alfalfa nodule invaded by wild type S. meliloti 1021 that does not express GUS (subjected to the same staining procedure as B–F). (Roots in B, C, and D were inoculated with strain SMc00911. Xsd1. Roots in E and F were inoculated with strain SMc00911.original.) Nodules were stained for 3.75 hr after 5 weeks of growth post-inoculation. Scale bars correspond to 0.1 mm. Panel G shows SMc00911-controlled GUS expression in S. meliloti grown on solid LBMC medium. Wild type S. meliloti 1021 is shown as a negative control for GUS expression and a strain carrying the same GUS insertion plasmid in the greA gene is shown as a positive control for GUS expression in free-living cells. Strain SMc00911.original and a ϕM12 transductant of this strain were tested on plants.