The NAM1 protein (NAM1p), which is selectively required for cox1, cytb and atp6 transcript processing/stabilisation, is located in the yeast mitochondrial matrix

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Abstract
The NAM1 nuclear gene was shown to control the stability and/or processing of mitochondrial transcripts of the cytochrome b, cytochrome oxidase subunit I and ATP synthase subunit VI genes [Groudinsky O., Bousquet I., Wallis M. G., Slonimski P. P. & 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 an antigen to raise polyclonal antibodies. These antibodies specifically recognise a 50‐kDa protein which purifies along with the mitochondria and corresponds to NAM1p. Submitochondrial localisation experiments show that NAM1p is a soluble protein, located interior to the mitoplasts. Matricial location is a strong argument in favour of a direct interaction of NAM1p with particular mitochondrial transcripts and leads us 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 occur.

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