Pathway Analysis of Sulfur Containing Amino Acids
Genes from Arabidopsis and potato and, when appropriate, E. coli involved in cysteine and methionine biosynthesis have also been cloned, including various isoforms of O-acetylserine (thiol)-lyase, the enzyme that converts O-acetylserine to cys; ATP-sulfurylase, the enzyme activating the inert sulfate through binding to ATP; and serine acetyltransferase (SAT), the enzyme catalysing the activation of serine to O-acetylserine. We manipulated the expression of these genes in an attempt to create conditions in which flux to either cysteine or methionine is increased.
For example, the over-expression of SAT using an E. coli gene targeted to plastids resulted in cysteine and glutathione (a tripeptide containing glutamic acid, cysteine, and glycine) levels almost twice as high as usual. By blocking the competing pathway to threonine using the partial antisense inhibition of threonine synthase in Arabidopsis and potato, we were able to increase leaf and tuber methionine levels significantly. Moreover, analysis of these transformants made it clear that there are species-specific differences in the regulation of methionine biosynthesis.
Our results in Nicotiana plumbaginifolia and potato have established the essential, but not rate-limiting, role of CbL in plant methionine biosynthesis. Furthermore, we found that regulation at the level of CgS differs between the plant species Arabidopsis and potato. Our objective now is to deepen our understanding of the regulation of methionine biosynthesis and to exploit what we learn in order to improve the nutritional quality of crop plants, which is largely determined by methionine content.
Cysteine biosynthesis represents the essential step in the incorporation of inorganic sulfide to organic sulfur in plants. In order to gain insight into the control mechanisms involved in cysteine biosynthesis, we are isolating and studying the involved genes and their promoters, including genes coding for O-acetylserine(thiol)-lyase (OAS-TL), the enzyme which converts O-acetylserine to cysteine, and serine acetyltransferase (SAT), the enzyme catalysing the activation of serine to O-acetylserine.
In addition, spatial and developmental aspects of regulation are investigated with respect to gene expression and enzyme activity. We are manipulating the expression of various genes in transgenic potato plants in an attempt to create conditions in which flux to either cysteine or methionine is increased. For example, the heterologous over-expression of an E. coli SAT gene targeted to plastids resulted in a doubling of both cysteine and glutathione (a tripeptide containing glutamic acid, cysteine, and glycine that is involved in stress tolerance) levels. However, these alterations had no effect on outward plant appearance or on the expression and enzymatic activity of OAS-TL. This example demonstrates the importance of SAT in plant cysteine biosynthesis and shows that the accumulation of cysteine and related sulfur-containing compounds is limited by the supply of activated carbon backbones derived from serine. We are currently investigating this and other transgenic plants affected in cysteine and methionine biosynthesis in respect to sulfur assimilation and glutathione-mediated stress tolerance.
Despite the increase of reduced organic sulfur in our potato SAT over-producers, we did not observe an increase in methionine, although other groups reported methionine increases when using a similar approach in maize (Tsakraklides et al., 2002). Again, species specific differences, probably as a result of adaptation to specific environmental or physiological conditions, have to be taken into account, especially when generalising and transferring these data to plant breeding.
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