Bacteriocin small of Rhizobium leguminosarum belongs to the class of N-acyl-L-homoserine lactone molecules, known as autoinducers and as quorum sensing co-transcription factors

Abstract

small bacteriocin was isolated from the culture broth of the gram-negative bacterium Rhizobium leguminosarum, which forms symbiotic nitrogen-fixing root nodules on a number of leguminous plants. The structure of the molecule was elucidated by spectroscopic methods and identified as N-(3R-hydroxy-7-cis-tetradecanoyl)-L-homoserine lactone. The absolute configuration of both asymmetric carbon atoms in the molecule was determined by the use of the chiral solvating agents S-(؉)-and R-(؊)-2,2,2-trifluoro-1-(9-anthryl)-ethanol. small bacteriocin is structurally related to the quorum sensing co-transcription factors for genes from other bacteria such as Vibrio fischeri, Pseudomonas aeruginosa, Erwinia carotovora, and Agrobacterium tumefaciens which are involved in animal-microbe or plant-microbe interactions. The mechanism of regulation of such interactions by this kind of co-transcription factors is still unknown in R. leguminosarum.small bacteriocin (small) is produced by strains of all three biovars of Rhizobium leguminosarum and inhibits the growth of R. leguminosarum bv. viciae 248 and several other strains of this species which, like strain 248, contain a self-transmissible plasmid (12,17). Two genes located close to the transfer (tra) genes (17) on the Sym plasmid pRL1JI of strain 248 are responsible for the fact that this strain does not produce small (rps [repression production small] gene) and that it is sensitive to small (sbs [small bacteriocin sensitivity] gene) (12,17). When strain 248 is cured of its Sym plasmid, pRL1JI (as, for example, in strain RBL1390), it is insensitive to small and also produces small (12, 17). To our knowledge, small is produced only by R. leguminosarum strains, and in nonproducing strains of this species, a gene for small production like that in strain 248 is present (12,17). Therefore, the presence of small is considered a characteristic of this species. Since another typical property of this species is symbiotic root nodule formation on certain leguminous plants, it is possible that both properties are related. However, a strain with a Tn5 insertion in the small gene(s) could induce formation of normal root nodules, which shows that the small gene is not required for root nodule formation (16). However, this does not exclude an ecological link between small production and the interaction of Rhizobium spp. with plants, since other genes may complement the lost function or the function is not essential. In order to detect its biological significance, small was extracted from the bacterial culture medium with chloroform (16) and identified chemically. The small molecule appeared to contain an N-acyl homoserine lactone structure, as known from quorum sensing signal molecules, which function as co-transcription factors in bacteria that often interact with higher organisms (reviewed by Fuqua et al. [10]). The implications of this finding are discussed. MATERIALS AND METHODSBacterial strains and growth conditions. The bacterial strains used in this study were the small-sensitive strain...

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