The 73 S ribosome of Neurospora crassa is the native mitochondrial ribosome.

The effect of the inclusion of EDTA and of heparin, in media used in the isolation of mitochondria, on the mitochindrial ribosome has been investigated. 1. Mitochondria isolated from Neurospora crassa in the presence of EDTA contain only a single type of monomeric ribosome, viz. 73 S. 2. Mitochondria isolated in the presence of Mg 2÷ contain both 79-S and 73-S monomeric ribosomes. The heterogeneity of the ribosomes was demonstrated by (a) ultracentrifugation on sucrose gradients, (b) electron microscopy, (c) immunoprecipitat ion with antibodies against mitochondrial 73-S and 79-S cytoplasmic ribosomes, (d) gel electrophoresis of high and low molecular weight RNAs. 3. Inclusion of heparin in all media used for the isolation of mitochondria and ribosomes resulted in (a) dissociation of 73-S mitochondrial ribosomes into 50-S and 37-S subunits; (b) stabilization of 79-S cytoplasmic ribosomes; (c) in the case of mitochondria isolated in the presence of Mg 2÷ containing both 73-S and 79-S ribosomes, heparin causes the selective dissociation of the 73 S monosome to yield ribosomes containing only a single monomeric ribosome type, viz. 79 S. 4. It is concluded that (a) the 79-S ribosomes present in mitochondria isolated in the presence of Mg 2÷ are contaminating cytoplasmic ribosomes, (b) the 73-S ribosomes are the real functional mitochondrial ribosomes of Neurospora crassa.