The nuclear SUV3-1 mutation affects a variety of post-transcriptional processes in yeast mitochondria
Nucleic Acids Research
The nuclear SUV3-1 mutation affects a variety of post- transcriptional processes in yeast mitochondria
Heather Conrad-Webb 0
Philip S.Perlman 0
Hong Zhu 0
Ronald A.Butow 0
0 department of Molecular Genetics, The Ohio State University , Columbus, OH 43210-1292 and department of Biochemistry , The University of Texas Southwestern Medical Center , Dallas, TX 75235-9038 , USA
The SUV3-1 mutation was isolated earlier as a suppressor of a deletion of a conserved RNA processing site (dodecamer) near the 3' end of the vart gene. Previous studies indicate that the suppressor enhances translation of mutant var1 messages; unexpectedly, it also causes over-accumulation of excised intron RNA of the large rRNA gene intron and blocks cleavage at the dodecamer site within that intron. In this study most mitochondrlal genes in SUV3-1 and suv3 nuclear contexts are surveyed for changes in levels of mRNA, for interference with dodecamer cleavage and splicing and for levels of excised intron RNAs. SUV3-1 has little or no effect on the size or abundance of unspliced RNAs tested. It results, however, In a marked increase in the abundance of seven of eight excised group I intron RNAs tested, most of which are not detectable in wildtype (suv3) strains. The suppressor lowers levels of the cob and coxl mRNAs about 2 - 5 and 20-fold, respectively. The effect on coxl mRNA results from a decrease in the splicing of its intron 5/3. Despite the reduction in these mRNA levels, the amounts of coxl and cyt b polypeptides were close to wild-type levels in SUV3-1 cells. These data show that the suv3 gene plays a prominent role in post-transcriptional and translation events in yeast mitochondria.
INTRODUCTION
The expression of yeast mitochondria] genes is regulated primarily
through post-transcnptional processes. In many cases RNAs are
transcribed as multigenic precursors that must be processed to
form mature mRNAs, rRNAs and tRNAs ( I - 1 2 ) . Potential
points for control of mitochondrial gene expression include,
therefore, 5' and 3' RNA processing, splicing, and RNA stability.
There is, in addition, increasing evidence that mitochondrial gene
expression is controlled by nuclear genes at the level of translation
and, possibly, by control of assembly of mitochondria! protein
complexes. Several genes are needed for splicing of mitochondrial
* To whom correspondence should be addressed
introns (
13 -16
), for translation of specific mRNAs (
17 - 20
) and
one is required for stability of one mRNA (
21, 22
). Recent
evidence indicates that a heat shock protein, hsp60, plays an
important role in mitochondnal assembly processes (23).
We have recently described a dominant nuclear suppressor,
called SUV3-1, that affects several mitochondrial
posttranscnptional processes. SUV3-1 was originally isolated (
24
) as
a spontaneous suppressor of a varl gene mutant, PZ206 (
25
).
That mutant contains a deletion of a conserved 3' processing
sequence, 5'AATAATATTCTT-3' (
24
), at which the 3' end of
mature varl mRNA is formed. The PZ206 mutation causes the
synthesis of varl transcripts with aberrant 3' ends that are quite
unstable relative to the wild-type mRNA. Since PZ206 has no
detectable mitochondrial protein synthesis it appears that the novel
varl transcripts are also poorly translatable. SUV3-1 appears to
enhance the translation of those atypical varl transcripts, since
neither their abundance nor the location of their 3' termini is
altered (
24
).
The only other gene analyzed previously in the SUV3-1
background is the w intron of the 21S rRNA gene. That intron
contains a reading frame encoding a site-specific endonuclease
required in crosses for the quantitative transmission of the intron
to 21S rRNA gene alleles lacking the intron (
26-29
). The w
intron is one of a growing number of examples of mobile introns
that encode a recombinogenic endonuclease (30). Surprisingly,
it was found that the SUV3-1 mutation abolishes cleavage at the
dodecamer that follows the intron reading frame and causes a
striking overaccumulation of the excised intron RNA (
31
).
In this report we show that the effects of the SUV3-1 mutation
on mitochondrial RNA metabolism and gene expression are not
limited to the varl gene and 21S rRNA gene intron. SUV3-1
causes most other group I intron RNAs to accumulate but does
not alter 3' end processing of any of the mRNAs tested. It was
unexpected to find that SUV3-1 interferes with splicing of at least
one intron of the coxl gene and that that effect was chiefly
responsible for a dramatic reduction of the amount of coxl
mRNA; yet, the level of respiratory activity in such cells was
barely reduced. These results suggest a critical role for SUV3-1
and its wild-type allele, suv3, in nuclear control of mitochondrial
gene expression.
COXI
Y////////9W//////Z,
all
a!1+L
al2
al3
a!4
a!4
a!5a
1 Group I lntron
Group II lntron
bh M2
bl3
bW bl5
The nuclear genomes used in this study are from th (...truncated)