The availability of complete genome sequence data from both bacteria and 
eukaryotes provides information about the contribution of bacterial genes to the 
origin and evolution of mitochondria. Phylogenetic analyses based on genes 
located in the mitochondrial genome indicate that these genes originated from 
within the alpha-proteobacteria. A number of ancestral bacterial genes have also 
been transferred from the mitochondrial to the nuclear genome, as evidenced by 
the presence of orthologous genes in the mitochondrial genome in some species 
and in the nuclear genome of other species. However, a multitude of 
mitochondrial proteins encoded in the nucleus display no homology to bacterial 
proteins, indicating that these originated within the eukaryotic cell subsequent 
to the acquisition of the endosymbiont. An analysis of the expression patterns 
of yeast nuclear genes coding for mitochondrial proteins has shown that genes 
predicted to be of eukaryotic origin are mainly translated on polysomes that are 
free in the cytosol whereas those of putative bacterial origin are translated on 
polysomes attached to the mitochondrion. The strong relationship with 
alpha-proteobacterial genes observed for some mitochondrial genes, combined with 
the lack of such a relationship for others, indicates that the modern 
mitochondrial proteome is the product of both reductive and expansive processes.