Divalent cations promote the transfer of the IncP-7 plasmid pCAR1 among different Pseudomonas hosts

An incompatibility P-7 plasmid pCAR1 can be efficiently transferred among artificial microcosms in the presence of divalent cations Ca2+ and Mg2+. One on one mating assays between Pseudomonas strains with different plasmids showed that the promotion of transfer efficiency by divalent cations was also found in other plasmids including pB10 and NAH7, whereas the impacts were larger in IncP-7 plasmids. The impact on pCAR1 transfer altered by different pairs of donor and recipient strains, and the promotion of transfer efficiency was clearly detected between donors P. resinovorans CA10dm4 and P. fluorescens Pf0-1 to the recipients P. putida KT2440 and CA10dm4. Transcriptome analyses showed that genes on the pCAR1 did not respond to the presence of cations, including the tra/trh genes involved in its transfer. Notably, transcriptions of oprH genes, encoding a putative outer membrane proteins, in both of donor and recipient were commonly upregulated under the cation-limited condition. Transfer frequency of the pCAR1 to the transposon mutant of oprH in KT2440 was not promoted by the cations. This effect was partially recovered by the complementation with oprH gene, suggesting that OprH is involved in the promotion the pCAR1 transfer by divalent cations. Transcriptome analysis was perfoemed using Pseudomonas fluorescens Pf0-1(pCAR1) and Pseudomonas putida KT2440 mating in carbon-free (CF) buffer with/without cations. RNA maps of the donor or the recipient were used as controls. Biologically duplicated data were obtained.