ISRf1, a transposable insertion sequence from Sinorhizobium fredii

Sinorhizobium fredii strain HH103, a nitrogen-fixing bacterial symbiont of plants, contains an insertion sequence (IS) that can transpose into plasmid pMUS248 and activate a promoterless TcR gene that is normally not expressed. We have cloned and characterized this element, which we designate ISRf1. The IS is 1002 bp in length, contains a single 513-bp open reading frame (ORF), is flanked by imperfect 36-bp terminal inverted repeats, and creates 5-bp target duplications. Two copies of ISRf1 are present in the genome of HH103, but it is absent from 12 other Sinorhizobium and Rhizobium strains. The element transposes at a frequency of 2.7 x 10(-6) per generation per cell.