Mitochondrial Mg2+ homeostasis is critical for group II intron splicing in vivo

The product of the nuclear MRS2 gene, Mrs2p, is the only candidate splicing factor essential for all group II introns in mitochondria of the yeast Saccharomyces cerevisiae. It has been shown to be an integral protein of the inner mitochondrial membrane, structurally and functionally related to the bacterial CorA Mg2+ transporter. Here we show that mutant alleles of the MRS2 gene as well as overexpression of this gene both increase intramitochondrial Me2+ concentrations and compensate for splicing defects of group II introns in mit(-) mutants M1301 and B-loop. Yet, covariation of Mg2+ concentrations and splicing is similarly seen when some other genes affecting mitochondrial Mg2+ concentrations are overexpressed in an mrs2 Delta mutant, indicating that not the Mrs2 protein per se but certain Me, concentrations are essential for group II intron splicing. This critical role of Mg2+ concentrations for splicing is further documented by our observation that pre-mRNAs, accumulated in mitochondria isolated from mutants, efficiently undergo splicing in organello when these mitochondria are incubated in the presence of 10 mM external Me, (mit- M1301) and an ionophore (mrs2 Delta). This finding of an exceptional sensitivity of group II intron splicing toward Mg2+ concentrations in vivo is unprecedented and raises the question of the role of Mg2+ in other RNA-catalyzed reactions in vivo. It explains finally why protein factors modulating Mg2+ homeostasis had been identified in genetic screens for bona fide RNA splicing factors.