Functional characterization of the CDF transporter SMc02724 (SmYiiP) in Sinorhizobium meliloti: Roles in manganese homeostasis and nodulation

Abstract

In bacteria, membrane transporters of the cation diffusion facilitator (CDF) family participate in Zn2+,Fe2+,Mn2+,Co2+and Ni2+homeostasis. The functional role during infection processes for several members has been shown tobe linked to the specificity of transport.Sinorhizobium melilotihas two homologous CDF genes with unknowntransport specificity. Here we evaluate the role played by the CDF SMc02724 (SmYiiP). The deletion mutant strainof SmYiiP (ΔsmyiiP) showed reduced in vitro growthfitness only in the presence of Mn2+.IncubationofΔsmyiiPand WT cells with sub-lethal Mn2+concentrations resulted in a 2-fold increase of the metal only in the mutantstrain. Normal levels of resistance to Mn2+were attained by complementation with the gene SMc02724 underregulation of its endogenous promoter. In vitro, liposomes with incorporated heterologously expressed pure pro-tein accumulated several transition metals. However, only the transport rate of Mn2+was increased by imposing atransmembrane H+gradient. Nodulation assays in alfalfa plants showed that the strainΔsmyiiPinduced a lowernumber of nodules than in plants infected with the WT strain. Our results indicate that Mn2+homeostasis inS. melilotiis required for full infection capacity, or nodule function, and that the specificity of transport in vivo ofSmYiiP is narrowed down to Mn2+by a mechanism involving the proton motive force.