A putative magnesium transporter AtMRS2-11 is localized to the plant chloroplast envelope membrane system

Abstract The AtMRS2 family of 10 Arabidopsis genes (and one pseudo-gene) belongs to a eukaryote subset of the CorA super-family of magnesium transporters. Of nine members of the Arabidopsis family expressed in yeast, three conferred the capability for magnesium uptake, with AtMRS2-11 the most effective. GFP fusions indicated that several members of the family were localized in yeast cells to regions other than the plasma membrane. Expression of the AtMRS2-11 gene was examined in plants. The protein is predicted to have a chloroplast leader peptide, and Western analysis showed it to be enriched in isolated chloroplasts. An AtMRS2-11:eGFP fusion protein expressed in transgenic plants was localized to the envelope membrane system of chloroplasts. The promoter of AtMRS2-11 is predicted to contain binding sites for transcription factors that are involved in light regulation. RT-PCR demonstrated that the RNA transcript levels of this gene show diurnal cycling, with a two-fold higher steady-state expression in the light. No change in steady-state expression was observed in etiolated seedlings during the first 2 h of light exposure. Expression of a GUS gene driven by a 2-kb AtMRS2-11 promoter fragment showed expression only in the tops of the plants, with highest staining in young leaves. In sections of leaf tissue, expression was evident in the spongy mesophyll and stomata cells. Overexpression of the AtMRS2-11 gene led to increased accumulation of protein but the plants showed no detectable phenotype. The results are consistent with the AtMRS2-11 gene playing a role in magnesium transport into chloroplasts/plastids.