Divalent cations increase the conjugation efficiency of the incompatibility P-7 group plasmid pCAR1 among different Pseudomonas hosts.

The incompatibility (Inc) P-7 group plasmid pCAR1 can be efficiently transferred among bacteria in 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 conjugation efficiency by divalent cations was exhibited in other plasmids, including pB10 and NAH7; however, this effect was larger in IncP-7 plasmids. The impact on pCAR1 conjugation differed according to donor-recipient pairs, and conjugation efficiency promotion was clearly detected between the donors P. resinovorans CA10dm4 and P. fluorescens Pf0-1 and the recipients P. putida KT2440 and CA10dm4. Transcriptome analyses showed that pCAR1 gene expression did not respond to cation changes, including the tra/trh genes involved in its transfer. However, the transcription of oprH genes, encoding putative outer-membrane proteins in both the donor and the recipient, were commonly upregulated under cation-limited conditions. The conjugation frequency of pCAR1 in the KT2440 oprH mutant was found not to respond to cations. This effect was partially recovered by complementation with the oprH gene, suggesting that OprH is involved in the increase of pCAR1 conjugation efficiency by divalent cations.