Mitochondrial ribosomes comprise the most diverse group of ribosomes known. The 
mammalian mitochondrial ribosomes (55S) differ unexpectedly from bacterial (70S) 
and cytoplasmic ribosomes (80S), as well as other kinds of mitochondrial 
ribosomes. The bovine mitochondrial ribosome has been developed as a model 
system for the study of human mitochondrial ribosomes to address several 
questions related to the structure, function, biosynthesis and evolution of 
these interesting ribosomes. Bovine mitochondrial ribosomal proteins (MRPs) from 
each subunit have been identified and characterized with respect to 
individuality and electrophoretic properties, amino acid sequence, topographic 
disposition, RNA binding properties, evolutionary relationships and reaction 
with affinity probes of ribosomal functional domains. Several distinctive 
properties of these ribosomes are being elucidated, including their antibiotic 
susceptibility and composition. Mammalian mitochondrial ribosomes lack several 
of the major RNA stem structures of bacterial ribosomes but they contain a 
correspondingly higher protein content (as many as 80 proteins), suggesting a 
model where proteins have replaced RNA structural elements during the evolution 
of these ribosomes. Despite their lower RNA content they are physically larger 
than bacterial ribosomes, because of the 'extra' proteins they contain. The 
extra proteins in mitochondrial ribosomes are 'new' in the sense that they are 
not homologous to proteins in bacterial or cytoplasmic ribosomes. Some of the 
new proteins appear to be bifunctional. All of the mammalian MRPs are encoded in 
nuclear genes (a separate set from those encoding cytoplasmic ribosomal 
proteins) which are evolving more rapidly than those encoding cytoplasmic 
ribosomal proteins. The MRPs are imported into mitochondria where they assemble 
coordinately with mitochondrially transcribed rRNAs into ribosomes that are 
responsible for translating the 13 mRNAs for essential proteins of the oxidative 
phosphorylation system. Interest is growing in the structure, organization, 
chromosomal location and expression of genes for human MRPs. Proteins which are 
essential for mitoribosome function are candidates for involvement in human 
genetic disease.