A member of a novel Arabidopsis thaliana gene family of candidate Mg2+ iontransporters complements a yeast mitochondrial group II intron-splicing mutant

Autocatalytic activity of some group II introns has been demonstrated in vi tro, but helper functions such as the yeast MRS2 protein are essential for splicing in vivo. In our search for such helper factors in plants, we pursu ed the cloning of two Arabidopsis thaliana homologues, atmrs2-1 and atmrs2- 2. Atmrs2-1, but not atmrs2-2, complements the yeast deletion mutant of mrs 2, and this is congruent with the prediction of two adjacent transmembrane stretches in AtMRS2-1 and yeast MRS2 but not in AtMRS2-2. This complementat ion depends on fusion of the native yeast mitochondrial import sequence to atmrs2-1. A differing, non-mitochondrial, cellular targeting in Arabidopsis is supported by the analysis of green fluorescent protein fusion construct s after transient transformation into plant protoplasts. Further members of what now appears to be a family of 10 mrs2 homologues are identified in th e Arabidopsis genome. Similarity searches with the psi-blast algorithm in t he protein database fail to identify homologues of this novel gene family i n any eukaryotes other than yeasts, but do identify its distant relatedness to the corA group of bacterial magnesium transporters. In line with this o bservation, intramitochondrial magnesium concentrations are indeed restored to wild-type levels in the yeast mutant on complementation with atmrs2-1.