Two plasmids from the bean-nodulating Sinorhizobium fredii strain GR64 regulate each other's conjugation genes

LAURA CERVANTES; GONZALO A. TORRES TEJERIZO; EUNICE LÓPEZ-FUENTES; SUSANA BROM

Cuatro Ciénegas, Coahuila

Congreso; III Congreso de Bioquímica y Biología Molecular de Bacterias; 2013

Sociedad Mexicana de Bioquímica

Sinorhizobium fredii GR64 was isolated from bean nodules in Granada, Spain. This strain contains three plasmids in addition to the chromosome: pSfr64a (183 Kb), the symbiotic plasmid (pSym) pSfr64b (460 Kb), and megaplasmid pSfr64c (>1000 Kb). In previous work, we analyzed the conjugative transfer ability of these plasmids. We found that pSfr64 is transmissible at high frequency, that this tranfer depends on quórum-sensing (QS) regulation mediated through a TraR transcriptional regulator, and that transfer of the pSym requires the presence of pSfr64a. A recently completed  draft of the whole genome sequence of GR64 showed that, in addition to the transfer genes present in pSfr64a, the pSym also contains a complete set of tra  genes. This raises the question: why is tranfer of the pSym dependent on  the presence of pSfr64a, if it contains all required tra  genes?             To answer this question we obtained derivatives with mutations in the regulatory genes from both plasmids and analyzed their conjugative phenotype. Also, we constructed transcriptional fusions to a reporter gene, from the regulatory genes, and determined their expression in the wild type and mutant backgrounds.             The results showed that the TraIa-TraRa complex encoded in pSfr64a induces expression of the transfer genes encoded in this plasmid, while the TraIb-TraRb complex encoded in the pSym regulates the genes from pSfr64b. However, expression of the pSym encoded traRb depends on the pSfr64a encoded TraRa, while expression of traRa requires the pSym encoded TraRb.             We conclude that the expression of the genes required for DNA transfer and mating-pair formation localized in each plasmid are regulated by the TraI-TraR complex encoded in the same plasmid, nevertheless, a regulatory circuit is formed among both plasmids, because the expression of the transcriptional regulator TraRa depends on traRb, and viceversa.