INSTITUTO DE BIOTECNOLOGIA Y BIOLOGIA MOLECULAR
Unidad Ejecutora - UE
congresos y reuniones científicas
Rhizobial strains employ different strategies for conjugative transfer regulation
BROM, S.; CERVANTES, L.; LÓPEZ-FUENTES, E.; ALTHABEGOITI, M. J.; LAGARES, A.; PISTORIO, M.; ROMERO, D.
Congreso; III Congreso de Bioquímica y Biología Molecular de Bacterias; 2013
Sociedad Mexicana de Bioquímica
Rhizobia are soil bacteria, able to fix nitrogen in symbiosis with the roots of leguminous plants. Their genomes usually contain a chromosome and a variable number of plasmids. One of these, the symbiotic plasmid (pSym), carries most of the genes requiered for nodulation and nitrogen fixation. To better understand the diversification of rhizobia, we aim to compare the mechanisms allowing conjugative transfer (CT) of their plasmids. We have focused on bean-nodulating strains belonging to different species: Rhizobium etli CFN42 (México), Sinorhizobium fredii GR64 (Spain), and Rhizobium sp. LPU83 (Argentina). These strains contain conjugative plasmids: pRet42a, pSfr64a and pLPU83a. The three plasmids have Dtr and Mpf genes, regulated by quórum-sensing (QS). However, each plasmid implements this regulation through different pathways. (1) CT of pRet42a is controlled through the transcriptional regulators TraRa and CinRa, together with the AHL-synthase TraIa, all encoded in the plasmid. Additionally, Orf 163, encoding a hypothetical conserved protein (hcp) ensures expression of the CT machinery, by induction of traI and repression of AHL-synthase genes located elsewhere in the genome. (2) In pSfr64a, induction of tra genes depends on traIa-traRa genes located in the plasmid, nevertheless, their expression is modulated by a TraIb-TraRb complex encoded in the pSym (pSfr64b). Also, a hcp encoding gene (orf 147 or trrA) was found to be required for CT. (3) pLPU83a lacks a traI gene, but carries a traR, which is required for its CT, suggesting that this regulator may be independendent of AHL-synthase. When pLPU83a is introduced into R.etli, it utilizes the regulatory machinery of plasmid pRet42a for its CT. Another interesting feature of these plasmids, is their ability to promote CT of the pSyms from their respective strains, also through different mechanisms: pRet42a from R. etli, is able to cointegrate with the pSym (pRet42d), by homologous or site-specific recombination, and thus promote its CT through donation. In contrast, in GR64, pSfr64a and the pSym (pSfr64b) depend on each other for mobilization. As mentioned above, pSfr64a CT requires TraIb-TraRb from the pSym. This complex also controls the Dtr and Mpf genes located on the pSym, but induction of traIb and traRb depends on the pSfr64a encoded TraRa, forming a regulatory circuit between both plasmids. In LPU83, CT of the pSym occurs at very low levels, but is also pLPU83a-dependent. The mechanism for this CT induction has not been elucidated. Acknowledgements: to DGAPA for PAPIIT grant IN203199, and to CONACYT-CONICET for México-Argentina bilateral cooperation grant Nº 188967.