Thermoplasma acidophilum, a thermophilic mycoplasma, has several unusual 
features suggesting a possible relationship to eukaryotic cells. One feature is 
a histone-like protein that is associated with the DNA, condensing it into 
subunits similar to those in eukaryotic chromatin. A second feature is an 
association of cytoplasmic proteins that resembles eukaryotic actin and myosin. 
These two components are widely distributed in different groups of eukaryotic 
cells, but are typically lacking in prokaryotic cells. Furthermore, T. 
acidophilum lacks cytochromes and respires by enzymes that apparently are not 
coupled to oxidative phosphorylation. This primitive type of respiration 
resembles that of microbodies, another feature which is represented in the 
cytoplasm of all groups of eukaryotic cells. Furthermore, since T. acidophilum 
lacks a cell wall and appears to have a primitive correlate of endocytosis, it 
would appear to be mechanically capable of acquiring a symbiotic mitochondrion. 
Thus, our observations are consistent with the symbiotic hypothesis for the 
origin of eukaryotic cells. We suggest that an organism similar to T. 
acidophilum was the host cell for the original symbiosis, becoming the nucleus 
and cytoplasm of modern eukaryotic cells.