The catabolic sequence from protein to amino acids, and then the breakdown of amino acids to ammonia, has now been characterized fairly well in terms of reactions and microbes. To date, however, relatively few studies have probed the reactions at the molecular/regulatory level, certainly in comparison with the flood of molecular studies on fibrolytic digestion. Given the significance of Prevotella spp. in rumen N metabolism, sequencing a Prevotella genome now seems overdue. Analysis of how the genetic information for catabolic N metabolism is organized at the chromosomal level may lead to new ideas about slowing the catabolic flux. 'Natural' means, new plants or plant extracts for example, of regulating the catabolic flux also require more attention.
The issue of selective supplementation of ruminant diets with the amino acids that limit microbial growth and ruminal fermentation remains to be resolved, if indeed a solution is possible. That there is no single amino acid which limits rumen fermentation is now very clear. This review has summarized the results of different studies which looked into the biosynthesis of individual amino acids in ruminal microorganisms. Whether a subset of amino acids will be found that can achieve the stimulatory capability of the complete mixture seems improbable. The implications of the information that de novo amino acid synthesis has different regulatory characteristics in microbes with different functions - the de novo syntheses most sensitive to the presence of preformed amino acids were phenylalanine for cellulolytic bacteria, proline for non-cellulolytic bacteria, and lysine for the fungi - are not yet clear and should be investigated further.
Recent mechanistic models have taken many factors regarding the quantitative aspects of ruminal fermentation into account, but they have not matched with the productivity of ruminants (Sauvant, 1997). One of the problems is that different species respond differently to various N sources, reflecting the diversity of the microbial ecosystem in the metabolism of amino acids. This factor alone could affect the efficiency of ruminal fermenta tion and the outcome of the models. Molecular techniques based on 16S rDNA analysis of microbial ecosystems offer a unique opportunity to provide population analysis of the microbial ecosystem under dietary conditions (Teather et ai, 1997). The new information about the concentration-dependence of amino acid and ammonia assimilation also further complicates nutritional models. Whether mechanistic nutritional models need to embrace such detail is not a judgement the present authors feel qualified to make. The challenge will be enormous.
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