Cytoplasmic Type 80 S Ribosomes Associated with Yeast Mitochondria

Abstract

Abstract Mitochondria isolated from starved or stationary phase cells (strain SB-1) have 40 to 60 % fewer associated cytoplasmic ribosomes than mitochondria isolated from log phase cells. A similar reduction in the amount of associated ribosomes was obtained when the initiation of cytoplasmic protein synthesis was inhibited using a temperature-sensitive yeast mutant. A change in the amount of mitochondria-bound cytoplasmic ribosomes was correlated directly with a change in the isopyknic density of the isolated mitochondria; moreover, the amount of cytoplasmic ribosomes associated with mitochondria could be predicted from the mitochondrial density. Mitochondria prepared from log phase cells contain 7.6 A260 units of cytoplasmic rRNA per mg of protein and will bind in vitro no more than 0.4 A260 unit of cytoplasmic ribosomes per mg of mitochondrial protein. From these results we calculate that only about 5 % of the ribosome binding sites are vacant in these mitochondrial preparations. On the other hand, mitochondria obtained from starved cells have only 4.7 A260 units of cytoplasmic rRNA per mg of protein and have a considerably greater capacity to bind ribosomes than mitochondria isolated from log phase cells. The binding of free cytoplasmic polysomes or cytoplasmic polysomes extracted from mitochondria to a preparation of EDTA-washed mitochondria was completely inhibited by either 10-4 m aurintricarboxylic acid or 350 mm KCl. However, the addition of 10-3 m aurintricarboxylic acid or 350 mm KCl to the medium used for the isolation of mitochondria had little effect on the amount of cytoplasmic rRNA recovered with the mitochondria. These data support the conclusion that cytoplasmic ribosomes are bound to yeast mitochondria in vivo.