pHNC3, a suicide plasmid encoding luxAB and scaB genes in Tn5,can be transfered into Rhizobium through tri-parental mating and thenthe Rhizobium is labelled by luxAB. Since the levansucrasc activitycoded by sacB gene can cause the sucrose sensitivity on Gram-negativebacteria,the sacB gene allowed direct selection for the deletion or curingof plasmide, because only strains which no longer contain an activesacB gene are able to grow on media containing sucrose. This negativeselecting model can be used for direct labelling the mega-plasmid ofRhizobium,or even getting Rhizobium deriyatives partial deleted or curedtheir mega-plasmids.
