New determinants in the regulation of conjugative plasmid transfer in rhizobia

LUCAS CASTELLANI; MARIANO PISTORIO; SUSANA BROM; JULIET NILSSON; GONZALO A. TORRES TEJERIZO

Seattle

Conferencia; Plasmid Biology 2018; 2018

Plasmids are widely found in rhizobia, a group of bacteria that are able to establish symbioticrelationships with the roots of legumes. Three main types of conjugative transfer (CT) regulationof these elements have been described [1]. The most prevalent is through Quorum-sensing (QS),mediated by the interaction of TraR and its cognate acyl-homoserine lactone (HSL). We haveshown that in a subgroup of rhizobial plasmids (represented by pLPU83a from Rhizobiumfavelukesii LPU83), the mechanisms that regulate their CT needs TraR, but not HSL [2]. Thesefeatures make pLPU83a an interesting plasmid for studying novel mechanism of CT regulation.We analyzed three in-tandem genes that encode for hypothetical proteins located betweenMpf and Dtr in pLPU83a. Mutation and complementation of the genes revealed that two of themare involved in CT regulation, that homologues are present in other bacteria, mainlyAlphaproteobacteria, and that they seem to have evolved together. One of the genes(pLPU83_0146) partially affects CT, while the other (pLPU83_0148), is indispensable, despite thatthey constitute an operon. RNAseq analysis of the mutants in pLPU83_0146 revealed that almostall the genes in the Dtr and Mpf are up-regulated, suggesting that pLPU83_0146 could act as arepressor, however, we do not know if the effect is direct o mediated by other molecules. OnlypLPU83_0148 is down-regulated in the mutant in pLPU83_0146, suggesting a self-regulatorymechanism of the operon. Mutation of pLPU83_0148 does not change the transcriptional profiles,indicating that it could play a structural role. Elucidation of the mechanisms that differentiallyregulate plasmid CT will be helpful to understand the boundaries of plasmid exchange in bacteria