pLPU83a, a model plasmid to study how different genomic backgrounds regulate the conjugative transfer

TORRES TEJERIZO, G. A.; CERVANTES, L.; ALTHABEGOITI, M. J.; PISTORIO, M.; ROMERO, D.; BROM, S.

Santander

Congreso; International Plasmid Biology Conference; 2012

International Society for Plasmid Biology

Horizontal Gene Transfer (HGT) is one of the main forces for microbial evolution. Bacterial HGT may be accomplished by transformation, transduction or conjugation. Conjugative transfer (CT) is considered one of the most efficient means of DNA exchange in prokaryotes, thus, special attention has been paid to the transfer of genetic material via plasmid conjugation. Rhizobia are common inhabitants of soil; also, they are able to form symbiotic associations with the roots of legumes, as nitrogen-fixing organisms. Usually, they harbor a variable number of plasmids. Some of these plasmids have been described as conjugative, others as mobilizable by accessory plasmids (1). Nevertheless, some plasmids have features that do not allow us to clearly define them as conjugative or mobilizable (2, 3). Plasmid pLPU83a from Rhizobium sp. LPU83, is able to perform CT from R. sp. LPU83 or Rhizobium etli CFN42, but is unable to transfer from plasmid-free Agrobacterium tumefaciens UBAPF2, suggesting that the genomic background affects the transfer ability. Aiming to understand how the CT of pLPU83a is regulated, we studied the elements present in different genomics backgrounds, which could be affecting conjugation. The CT of some rhizobial plasmids is regulated by Quorum-Sensing (QS) mediated by traR and traI genes. The cluster of Mpf/Dtr genes present in pLPU83a is very similar to the one present in pCFN42a of R. etli CFN42. Although a traR gene is present, the absence of traI is remarkable. Plasmid pLPU83a lacking a functional traR loses its ability to transfer, suggesting that CT of pLPU83a is regulated by QS. Plasmid pLPU83a is able to transfer from wild type R. etli CFN42. Analyzing pLPU83a CT from plasmid-cured derivatives of CFN42, we determined that pCFN42a is required for CT of pLPU83a from R. etli. Also, pCFN42a mutants unable to activate their own Dtr genes were unable to allow CT of pLPU83a. These results open 2 possibilities: that the Dtr of pCFN42a may process the oriT of pLPU83a, or that regulatory genes of pCFN42a activate the Dtr genes of pLPU83a. We have determined that the oriT of pLPU83a can be processed by the relaxase encoded in pCFN42a. However, the participation of other regulatory elements has not been discarded. Our results suggest that the CT phenotype (conjugative or mobilizable) depends on the interaction among elements from different replicons. Elucidation of the mechanisms that differentially regulate plasmid CT in different genomic backgrounds will be helpful to understand the boundaries of plasmid exchange in bacteria. 1. García de los Santos et al., World Journal Microbiol. Biotech. 12, 119 (1996). 2. Torres Tejerizo et al., Plasmid 64, 177 (Nov, 2010). 3. Cervantes et al., BMC Microbiol 11, 149 (Jun 25, 2011).