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

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 to be linked to the specificity of transport. Sinorhizobium meliloti has two homologous CDF genes with unknown transport specificity. Here we evaluate the role played by the CDF SMc02724 (SmYiiP). The deletion mutant strain of SmYiiP (Delta smyiiP) showed reduced in vitro growth fitness only in the presence of Mn2+. Incubation of Delta smyiiP and WT cells with sub-lethal Mn2+ concentrations resulted in a 2-fold increase of the metal only in the mutant strain. Normal levels of resistance to Mn2+ were attained by complementation with the gene SMc02724 under regulation of its endogenous promoter. In vitro, liposomes with incorporated heterologously expressed pure protein accumulated several transition metals. However, only the transport rate of Mn2+ was increased by imposing a transmembrane H+ gradient. Nodulation assays in alfalfa plants showed that the strain Delta smyiiP induced a lower number of nodules than in plants infected with the WT strain. Our results indicate that Mn2+ homeostasis in S. meliloti is required for full infection capacity, or nodule function, and that the specificity of transport in vivo of SmYiiP is narrowed down to Mn2+ by a mechanism involving the proton motive force. (C) 2014 Elsevier B.V. All rights reserved.