Pathway Analysis of Sulfur Containing Amino Acids

To learn about the control mechanisms involved in the biosynthesis of sulfur-containing amino acids, we are isolating and studying genes involved and their promoters. Methionine is synthesised from cysteine and phosphohomoserine via the enzymes cystathionine gamma-synthase (CgS), cystathionine beta-lyase (CbL), and methionine synthase (MS); we have cloned and characterised these three genes in potato.

Sulfate Uptake and Assimilation

Since sulfur is a required macronutrient for plants, sulfur uptake and assimilation is a crucial determinant of plant growth and vigour, crop yield, and even of the ability of plants to resist pests and cope with various stresses. Inorganic sulfate is taken up through the root system either as sulfate from the soil or, to a lesser extent, as volatile sulfur compounds from the air. Furthermore, plants can emit volatile sulfur compounds to the atmosphere, thus contributing to the overall sulfur cycle. In a cascade of enzymatic steps, inorganic sulfur is converted to the nutritionally important sulfur-containing amino acids, cysteine and methionine. The enzymes involved in assimilation are highly regulated at both gene and protein levels. This regulation is further complicated by the subcellular localisation of enzymes as well as by developmental and spatial activity patterns, which differ among plant species. Sulfate, which is relatively inert, is activated through binding to ATP, a reaction catalysed by ATP-sulfurylase (ATP-S).

Though mammals are able to synthesize cysteine from methionine, they are still dependent on the uptake of either sulfur-containing amino acid. Our investigations focus on fundamental questions about cysteine and methionine biosynthesis and on the possibility of engineering a crop plant of improved nutritional quality that is enriched in these amino acids. The molecular engineering of amino acid biosynthesis serves as a tool for investigating pathways at molecular, biochemical, and physiological levels. Transgenic plants are subjected to detailed metabolite analyses with respect to free amino acid composition and pathway intermediates.

R Höfgen, O Kreft, L Willmitzer, H Hesse (2001) Manipulation of thiol contents in plants. Amino Acids 20, 3, 291-299

H Hesse, O Kreft, S Maimann, M Zeh, L Willmitzer, R Höfgen (2001) Approaches towards understanding methionine biosynthesis in higher plants. Amino Acids 20, 3, 281-289

H Hesse, K Harms, D Klonus, P von Ballmoos, C Brunold, L Willmitzer, and R Höfgen (2000) Molecular engineering of sulfur assimilation in higher plants. In: Plant Sulfur Research in Europe, Landbauforschung Völkenrode, Sonderheft 218, 55-59

H Hesse, K Harms, P von Ballmoos, C Brunold, L Willmitzer, and R Höfgen (2000) Serine acetyl-transferase: A bottleneck for cysteine and glutathione synthesis?. In: Sulfur Nutrition and Sulfur Assimilation in Higher Plants: molecular, biochemical and physiological aspects. Brunold, C., Rennenberg, H., De Kok, L.J., Stulen, I. and Davidian, J.C. (eds), Paul Haupt, Bern, 2000, pp. 321-323

R Höfgen and H Hesse (2000) Molecular Engineering of Sulfate Assimilation. In: Sulfur Nutrition and Sulfur Assimilation in Higher Plants: molecular, biochemical and physiological aspects. Brunold, C., Rennenberg, H., De Kok, L.J., Stulen, I. and Davidian, J.C. (eds), Paul Haupt, Bern, 2000, pp. 109-124

AP Casazza, A Basner, R Höfgen, and H Hesse (2000) Expression of threonine synthase from Solanum tuberosum L. is not metabolically regulated by photosynthesis-related signals or by nitrogenous compounds. Plant Sci., 157, 43-50

Further reading

MJ Hawkesford (2003) Transporter gene families in plants: the sulphate transporter genefamily - redundancy or specialization? Physiologia Plantarum. 117(2)155-163 Sulfur Nutrition and Sulfur Assimilation in Higher Plants: molecular, biochemical and physiological aspects. Brunold, C., Rennenberg, H., De Kok, L.J., Stulen, I. and Davidian, J.C. (eds), Paul Haupt, Bern, 2000, pp. 321-323