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Vol. 15, No. 17, pp. 2229-2237, September 1, 2001
Vienna Biocenter, Department of Microbiology and Genetics,
University of Vienna, A-1030 Vienna, Austria
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 Mg2+ 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
mutant, indicating that not the Mrs2 protein per
se but certain Mg2+ 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 Mg2+ (mit
M1301) and an ionophore (mrs2
). 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.
[Key Words: Group II introns; RNA splicing; Mg2+; yeast; mitochondria; Mrs2p]
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