Acetohydroxyacid synthase(AHAS) is involved in the synthesis of branched-chain amino acids(BCAAs) in Arabidopsis.To investigate the effects of various domains of AHAS on the BCAAs synthesis,the point mutations harboring in the specific sites of the large and small units of AHAS were introduced by site-directed mutagenesis.The mutagened histidine-tagged units of AHAS were expressed individually in the bacterial hosts and the recombinant proteins were purified using Ni beads.The point mutated large and small units were reconstituted in vitro and the activities of holoenzymes were determined.Moreover,the effects of valine,which is one of the final end products of AHAS,on the activities of the mutated holoenzymes were also examined.The results showed that the G88D mutation in the small unit of AHAS abolished the final end product inhibition and the E305D or E482D mutation in the large unit decreased the activity of AHAS holoenzyme.The two mutations in the large unit displayed difference in the activity of AHAS and the E482D mutation presents the more effects than the E305D on the activity of AHAS.The results in this study suggest that the large unit interacts with the small unit in the AHAS and the various domains in the units of AHAS exhibit distinct functions.