The anaerobic fungi are an important component of the cellulolytic flora of the rumen (Orpin and Joblin, 1997). Ruminal fungi, unlike ruminal cellulolytic bacteria, are known to be proteolytic (Wallace and Joblin, 1985), which probably favours the disruption of the proteinaceous layer that prevents cellulolytic bacteria from gaining access to the secondary cell wall (Engles and Brice, 1985).
Rumen fungi are able to grow in media lacking preformed amino acids and therefore must be able to form the needed amino acids, but information on amino acid biosynthesis is lacking (Lowe et al., 1985). Amino acids, particularly aromatic amino acids, were stimulatory to growth (Orpin and Greenwood, 1986), however little published information is available concerning the amino acid metabolism of rumen fungi (Wallace et al., 1997).
Recently, a nutritional study using 15N-NH3 was undertaken with two predominant species of rumen fungi, Piromyces communis and Neocallimastix frontalis, to determine the influence of nitrogen source and concentration on de novo synthesis of amino acids (C. Atasoglu and R.J. Wallace, unpublished data). The proportions of cell-N and amino acids formed de novo from ammonia decreased as the concentration of peptides and amino acids increased, in a manner not dissimilar to the non-cellulolytic bacteria. Glutamate was the most highly enriched amino acid, implying that as in rumen bacteria glutamate dehydrogenase is the main pathway for ammonia uptake. De novo synthesis of lysine was much less than that of the other amino acids in both species, indicating preferential utilization of this amino acid. Phenylalanine was the second amino acid whose synthesis was affected most by the presence of peptides.
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