THE NAM1 MTF2 NUCLEAR GENE-PRODUCT IS SELECTIVELY REQUIRED FOR THE STABILITY AND/OR PROCESSING OF MITOCHONDRIAL TRANSCRIPTS OF THE ATP6 ANDOF THE MOSAIC, COX1 AND CYTB GENES IN SACCHAROMYCES-CEREVISIAE/
O. Groudinsky et al., THE NAM1 MTF2 NUCLEAR GENE-PRODUCT IS SELECTIVELY REQUIRED FOR THE STABILITY AND/OR PROCESSING OF MITOCHONDRIAL TRANSCRIPTS OF THE ATP6 ANDOF THE MOSAIC, COX1 AND CYTB GENES IN SACCHAROMYCES-CEREVISIAE/, MGG. Molecular & general genetics, 240(3), 1993, pp. 419-427
The NAM1/MTF2 gene was firstly isolated as a multicopy suppressor of m
itochondrial splicing deficiencies and independently as a gene of whic
h a thermosensitive allele affects mitochondrial transcription in orga
nello. To determine which step in mitochondrial RNA metabolism is cont
rolled in vivo by the NAM1 gene, mitochondrial transcripts of seven tr
anscription units from strains carrying an inactive nam1 = URA3 gene d
isruption in various mitochondrial genetic backgrounds were analysed b
y Northern blot hybridisations. In a strain carrying an intron-contain
ing mitochondrial genome, the inactivation of the NAM1 gene led to a s
trong decrease in (or total absence of) the mosaic cytb and cox1 mRNAs
and in transcripts of the atp6-rf3/ens2 genes, which are co-transcrib
ed with cox1. Neither the accumulation of unspliced cytb or cox1 pre-m
RNAs, nor that of excised circular intron molecules of ai1 or ai2 were
observed, but the abundance of the bi1 and ai7 lariats was comparable
to that observed in the wild-type strain, thus demonstrating that tra
nscription of the cytb and cox1 genes does occur. In strains carrying
the intron-less mitochondrial genome with or without the rf3/ens2 sequ
ence, wild-type amounts of cytb and cox1 mRNAs were detected while the
amount of the atp6 mRNA was always strongly decreased. The abundance
of transcripts from five other genes was either slightly (21S rRNA) or
not at all (cox2, cox3, atp9 and 15S rRNA) affected by the nam1 inact
ivation. This analysis leads to the conclusion that the NAM1 protein i
s not a general mitochondrial transcription factor, but rather is pred
ominantly and selectively required for the processing and/or for the s
tability of cytb and cox1 intron-containing pre-mRNAs and of the atp6
transcripts. Since the original intronic mutations suppressed by the a
mplification of the NAM1 gene are situated in stem-loop rich structure
s, we propose that the NAM1 protein is a stem-loop RNA-binding protein
that plays a role in determining RNA stability.