Methods for measuring cysteine and determining cysteine synthase activity

A method for identifying a test compound as a herbicide candidate, comprising:

• a) combining O-acetyl-L-serine, sulfide and cysteine synthase to form a reaction mixture in the presence and absence of said test compound;
• b) contacting said reaction mixture with a coumarin dye capable of conjugating with cysteine;
• c) subjecting the reaction mixture from step (b) to UV light and detecting fluorescent light emission;
• and
• d) comparing the amount of said fluorescent light emission in the presence and absence of said test compound;
• wherein a decrease in the amount of said fluorescent light emission in the presence of said test compound indicates that said test compound is a herbicide candidate.

A method for identifying compounds capable of selectively inhibiting plant, fungal and/or bacterial cysteine synthase activity, comprising:

• a) combining O-acetyl-L-serine, sulfide and a plant cysteine synthase to form a reaction mixture under conditions suitable for the production of cysteine;
• b) contacting said reaction mixture with a coumarin dye capable of conjugating with cysteine;
• c) subjecting the reaction mixture from step (b) to UV light and detecting fluorescent light emission,
• d) determining the inhibitory activity of the compound based on the fluorescent light emission,
• e) repeating steps a-d using a fungal or bacterial cysteine synthase, and
• f) identifying compounds that selectively inhibit plant, fungal or bacterial cysteine synthase.