Identification and characterization of insertion sequence ISRm1 in Rhizobium meliloti JJ1c10

ISRm1, an insertion sequence present in Rhizobium meliloti strain 1021, has been identified as the cause of the Nod⁻ phenotypes in two mutants of another strain, JJ1c10. The insertions were found to be at different sites, though only about 100 base pairs apart within the nodC gene. ISRm1 causes no mutations in the nifHDK gene region of strain JJ1c10, as it does at high frequency in strain 1021. In JJ1c10 ISRm1 inserted at high frequency into a region of the genome adjacent to copies of other reiterated DNA segments. The target region was not required for symbiotic nitrogen fixation.