whereas the catalytic pyrB subunits lost their activity at 65-70℃. Saturation curves for CP are Michaelian, both for the holoenzyme and the catalytic subunits (Km of 0.2 mM and 0.6 mM, respectively). Saturation curves for aspartate are sigmoidal for the holoenzyme and for the catalytic subunits (S_0.5 of 1 mM and 2 mM respectively), suggesting the existence of homotropic cooperative interactions between catalytic sites. The effectors ATP, CTP, GTP and UTP exert an inhibitory effect on the catalytic subunits but in contrast they activate the holoenzyme.

　

1. Van de Casteele, M., Demarez, M., Legrain, C., Glansdorff, N., and Pierard, A. (1990) J. Gen. Microbiol., 136, 1177-1183.

2. Legrain, C., Demarez, M., Glansdorff, N., and Pierard, A. (1995) Microbiology, 141, 1093- 1099.

3. Durbecq, V., Legrain, C., Roovers, M., Pierard, A., and Glansdorff, N. (1997) Proc. Natl. Acad. Sci., in press.

4. Roovers, M., Hethke, C., Legrain, C., Thomm, M., and Glansdorff, N. (1997) Eur. J. Biochem., 247, 1038-1045.

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