Ciliate protozoa are important inhabitants of the microbial ecosystem in the rumen. Despite the common idea that protozoa rely on bacteria for their amino acid requirements and do not use ammonia as a nitrogen source for protein synthesis (Nolan, 1993), there is some evidence for de novo synthesis of amino acids in protozoa (Williams and Coleman, 1992). 14C-labelled monosaccharides were incorporated into the protein of holotrichs (Williams and Harfoot, 1976); Williams, 1979), as was 14C-labelled sodium carbonate, which was incorporated into alanine, histidine, threonine, glutamate and aspartate (Harmeyer, 1965). Ciliates form lysine from diaminopimelic acid which is present in the cell wall peptidoglycan of the bacteria which they ingest (Onodera and Kandatsu, 1974; Onodera et al., 1974; Masson and Ling, 1986), and presumably many of the other amino acids are incorporated direct, after the digestion of bacteria, or are formed from pre-existing carbon skeletons derived from the bacteria.
Because the ciliate protozoa harbour bacteria in their cytoplasm, which are impossible to remove completely, it is difficult to distinguish genuine do novo amino acid biosynthesis from a secondary uptake via the biosynthetic activity of the cytoplasmic bacteria and the subsequent digestion of the bacteria by protozoal enzymes. Recently, however, a biosynthetic NAD-linked GDH was cloned from Entodinium caudatum, sequenced and the kinetic properties of its gene product measured (Eschenlauer et al., 1999). The cloned gene appeared to be biosynthetic because it had a low affinity for glutamate and a high affinity for ammonia, indicating that ciliate protozoa have the capability for ammonia assimilation. The quantitative significance of ammonia uptake by ciliate protozoa remains to be determined.
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