Mitochondrial Mg(2+) 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 Mg(2+) transporter. Here we show that mutant alleles of the MRS2 gene as well as overexpression of this gene both increase intramitochondrial Mg(2+) concentrations and compensate for splicing defects of group II introns in mit(-) mutants M1301 and B-loop. Yet, covariation of Mg(2+) concentrations and splicing is similarly seen when some other genes affecting mitochondrial Mg(2+) concentrations are overexpressed in an mrs2Delta mutant, indicating that not the Mrs2 protein per se but certain Mg(2+) concentrations are essential for group II intron splicing. This critical role of Mg(2+) 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 Mg(2+) (mit(-) M1301) and an ionophore (mrs2Delta). This finding of an exceptional sensitivity of group II intron splicing toward Mg(2+) concentrations in vivo is unprecedented and raises the question of the role of Mg(2+) in other RNA-catalyzed reactions in vivo. It explains finally why protein factors modulating Mg(2+) homeostasis had been identified in genetic screens for bona fide RNA splicing factors.