The structure of yeast phosphoglycerate kinase has been determined with data obtained from amino acid sequence, nucleotide sequence, and X-ray crystallographic studies. The substrate binding sites, as deduced from electron density maps, are compatible with known substrate specificity and the stereochemical requirements for the enzymic reaction. A carboxyl-imidazole interaction appears to be involved in controlling the transition between the open and closed forms of the enzyme.

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The first author acknowledges the support of EU postdoctoral fellowship CT940538.

Excessive ammonia production by rumen micro-organisms is a major cause of inefficient N retention by ruminants. The microorganisms which break down peptides and amino acids to ammonia were identified originally as typical major species of the rumen ecosystem (Bladen et al, 1961, Appl Microbiol, 9, 175-180) Four rumen-fistulated sheep received 500 g of hay, barley, molasses, fishmeal and vitaminsminerals (500, 299.5, 100, 91 and 9.5 g/kg DM) twice daily. Samples of rumen fluid were removed 2-3 h after feeding, strained, and diluted serially in the basal medium described by Chen and Russell (1990, Appl Environ Microbiol, 56, 2186-2192 Hence numbers of monensin-sensitive bacteria capable of growth on Trypticase were low, but they had a significant, although minor, role in ammonia production by the mixed population.