Mitochondrial Mg2+ homeostasis is critical for group II intron splicing invivo

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 Sac charomyces cerevisiae. It has been shown to be an integral protein of the i nner mitochondrial membrane, structurally and functionally related to the b acterial CorA Mg2+ transporter. Here we show that mutant alleles of the MRS 2 gene as well as overexpression of this gene both increase intramitochondr ial Me2+ concentrations and compensate for splicing defects of group II int rons in mit(-) mutants M1301 and B-loop. Yet, covariation of Mg2+ concentra tions and splicing is similarly seen when some other genes affecting mitoch ondrial Mg2+ concentrations are overexpressed in an mrs2 Delta mutant, indi cating that not the Mrs2 protein per se but certain Me, concentrations are essential for group II intron splicing. This critical role of Mg2+ concentr ations for splicing is further documented by our observation that pre-mRNAs , accumulated in mitochondria isolated from mutants, efficiently undergo sp licing in organello when these mitochondria are incubated in the presence o f 10 mM external Me, (mit- M1301) and an ionophore (mrs2 Delta). This findi ng of an exceptional sensitivity of group II intron splicing toward Mg2+ co ncentrations in vivo is unprecedented and raises the question of the role o f Mg2+ in other RNA-catalyzed reactions in vivo. It explains finally why pr otein factors modulating Mg2+ homeostasis had been identified in genetic sc reens for bona fide RNA splicing factors.