PROGRESS IN PROTOZOOLOGY
PROGRESS IN PROTOZOOLOGY
PROGRESS IN PROTOZOOLOGY
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in eukaryotes. It is about the same size as the others, and has about<br />
the same amino acid composition. The Tetrahymena molecule seemed<br />
to confirm the reputation of H4 as the most conservative protein molecule<br />
known. Thus far, three mammalian H4 molecules have been sequenced;<br />
no differences were found among the pig, the calf and the<br />
rat. The sea urchin differs from the mammals in one amino acid substitution;<br />
the pea plant differs in two amino acid replacements. But when<br />
Glover and Gorovsky (1979) began to sequence the Tetrahymena<br />
H4 molecule, the conservatism was shattered. The molecule has<br />
been only partially sequenced, but it already differs from mammalian<br />
H4 in at least 15 respects, including a deletion, an insertion, and many<br />
changes of amino-acid type. Tetrahymena has the most aberrant H4<br />
molecule thus far sequenced.<br />
Once again, this one item of information does not tell us very much<br />
about the relations among the ciliates or between them and other protists.<br />
It is consistent with a very early separation of the ciliates from<br />
the main root of the eukaryotes.<br />
Another protein very important in evolutionary considerations is<br />
cytochrome c. The evolutionary history of this protein is fascinating,<br />
because the protein appears in both eukaryotes and in eubacteria. Cytochrome<br />
c is a mitochondrial enzyme and is widely believed to have come<br />
into the eukaryotes along with the captured mitochondrial genetic<br />
system at the time the modern eukaryotes appeared. In any case, the<br />
ubiquity of the protein and the substantial comparative information<br />
concerning it make it a useful phylogenetic marker. G. E. Tarr (cited<br />
as personal communication in Rag an and Chapman 1978) has<br />
determined the amino acid sequence of cytochrome c from Tetrahymena,<br />
and has concluded that it is the most atypical eukaryotic cytochrome<br />
c thus far sequenced. In this case the evidence for a unique phylogenetic<br />
position is somewhat stronger than in the case of 5S RNA and histone<br />
H4, because more comparative data are available. At least Crithidia,<br />
Euglena and Physarum seem to be of later phylogenetic origin.<br />
One final conservative molecule, in this case a conservative protein<br />
of the plasma membrane, supports the evidence of the molecules previously<br />
considered. Nozawa and Nagao (1981) have reported at<br />
these meetings the primary structure of a Tetrahymena calmodulin.<br />
They review the evidence for homologies of the molecules compared<br />
and show that the Tetrahymena protein is the most distinctive of the<br />
calmodulins thus far studied.<br />
Inferences based on any o.ne of these molecules are of uncertain<br />
reliability, particularly because of the lack of an adequate comparative<br />
base. In conjunction, however, the four molecules allow a strong supposition<br />
that Tetrahymena, and coincidentally the ciliates generally, were<br />
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