Repression of Small bacteriocin excretion in Rhizobium leguminosarum and Rhizobium trifolii by transmissible plasmids

Abstract

All of the fast growing Rhizobium leguminosarum and R. trifolii strains studied, except four, excrete a small bacteriocin (small) into the culture medium. Only the four non-excreting strains harbour highly transmissible plasmids among which are the Sym plasmids pRL1JI (pea cross-inoculation group) and pRtr5a (clover cross-inoculation group). Small production genes were demonstrated in three of these strains and a plasmid function preventing excretion of small was present in all four strains (Rps). Two of the plasmids rendered the cells sensitive to small (Sbs). The plasmid functions Rps, Sbs, and medium bacteriocin production (Mep) on pRL1JI were expressed in R. leguminosarum, R. trifolii, R. phaseoli and A. tumefaciens. The formation of thick and short roots (Tsr) on Vicia sativa and nodulation (Nod) were also expressed in these hosts. No expression of above mentioned functions was found in R. meliloti.