THE NAM1 PROTEIN (NAM1P), WHICH IS SELECTIVELY REQUIRED FOR COX1, CYTB AND ATP6 TRANSCRIPT PROCESSING STABILISATION, IS LOCATED IN THE YEAST MITOCHONDRIAL MATRIX/

The NAM1 nuclear gene was shown to control the stability and/or proces sing of mitochondrial transcripts of the cytochrome b, cytochrome oxid ase subunit I and ATP synthase subunit VI genes [Groudinsky O., Bousqu et I., Wallis M. G., Slonimski P. P. and Dujardin G. (1993) Mol. Gen. Genet. 240, 419-427]. In order to better understand the mode of action of the NAM1 gene product, we have examined its intracellular fate. A fusion plasmid enabling bacterial over-expression of the corresponding protein-A-NAM1 cognate was constructed and subsequently employed as a n antigen to raise polyclonal antibodies. These antibodies specificall y recognise a 50-kDa protein which purifies along with the mitochondri a and corresponds to NAM1p. Submitochondrial localisation experiments show that NAM1p is a soluble protein, located interior to the mitoplas ts. Matricial location is a strong argument in favour of a direct inte raction of NAM1p with particular mitochondrial transcripts and leads u s to propose a model in which NAM1p could be an RNA-convoying protein stabilising and directing mitochondrial transcripts towards the inner face of the inner membrane where translation and assembly seem to occu r.