THE NAM1/MTF2 NUCLEAR GENE-PRODUCT IS SELECTIVELY REQUIRED FOR THE STABILITY AND/OR PROCESSING OF MITOCHONDRIAL TRANSCRIPTS OF THE ATP6 AND OF THE MOSAIC, COX1 AND CYTB GENES IN SACCHAROMYCES-CEREVISIAE

The NAM1/MTF2 gene was firstly isolated as a multicopy suppressor of mitochondrial splicing deficiencies and independently as a gene of which a thermosensitive allele affects mitochondrial transcription in organello. To determine which step in mitochondrial RNA metabolism is controlled in vivo by the NAM1 gene, mitochondrial transcripts of seven transcription units from strains carrying an inactive nam1 = URA3 gene disruption in various mitochondrial genetic backgrounds were analysed by Northern blot hybridisations. In a strain carrying an intron-containing mitochondrial genome, the inactivation of the NAM1 gene led to a strong decrease in (or total absence of) the mosaic cytb and cox1 mRNAs and in transcripts of the atp6-rf3/ens2 genes, which are co-transcribed with cox1. Neither the accumulation of unspliced cytb or cox1 pre-mRNAs, 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 transcription of the cytb and cox1 genes does occur. In strains carrying the intron-less mitochondrial genome with or without the rf3/ens2 sequence, 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 inactivation. This analysis leads to the conclusion that the NAM1 protein is not a general mitochondrial transcription factor, but rather is predominantly and selectively required for the processing and/or for the stability of cytb and cox1 intron-containing pre-mRNAs and of the atp6 transcripts. Since the original intronic mutations suppressed by the amplification of the NAM1 gene are situated in stem-loop rich structures, we propose that the NAM1 protein is a stem-loop RNA-binding protein that plays a role in determining RNA stability.