PROGRESS IN PROTOZOOLOGY

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