Mitochondrial Translation Products before and after Integration into the Mitochondrial Membrane in Neurospora crassa

# 1. Nascent translation products on mitochondrial ribosomes were
selectively labeled in vivo in the presence of cycloheximide with
radioactive leucine. They were isolated together with the
ribosomes. # 2. The labeled polypeptides show a high tendency to
aggregate and can only be kept in solution in the presence of
detergents such as dodecylsulfate. Also, mitochondrial ribosomes
carrying nascent peptide chains easily form aggregates. # 3. The
polypeptides adhering to mitochondrial monomeric ribosomes differ
from those adhering to polymeric ribosomes. Gel electrophoresis in
the presence if dodecylsulfate shows for the peptidyl transfer RNA
products at the monomer, an apparent molecular weight of 27000.
After removing the transfer RNA, an apparent molecular weight of
less than 10000 is registered. The peptides adhering to
mitochondrial polymeric ribosomes display a broad range of apparent
molecular weights. In contrast, translation products associated
with cytoplasmic monomeric and polymeric ribosomes all show quite
disperse molecular weights. # 4. Using gel-chromatographic analysis
no difference in the elution characteristics between translation
products associated with mitochondrial monomeric and polymeric
ribosomes was found. In both cases apparent molecular weights of
about 11000 were obtained. # 5. A kinetic study of the appearance
of mitochondrial translation products in the mitochondrial membrane
was carried out. A conversion process of products with lower
apparent molecular weights to those with higher apparent molecular
weights is observed. This suggests that mitochondrial ribosomes
form polypeptides which are modified during or after integration
into the membrane. # 6. The hypothesis is discussed that
mitochondria possess their own system of transcription and
translation, because the hydrophobic nature of the translation
products makes it necessary that they are formed inside the inner
mitochondrial membrane, into which they are integrated.