Several properties of the three acetohydroxy acid synthases of Escherichia coli have been compared in crude extracts. The three enzymes can be readily distinguished from each other. Acetohydroxy acid synthase I, the product of the iluB gene, has been purified to near homogeneity. The purification was made possible by the fact that the enzyme was maintained in buffers of a high ionic strength or in buffers containing glycerol. Density gradient centrifugation studies indicated that the enzyme exists as a dimer of subunits of similar (60,000) molecular weight in buffers containing glycerol with or without two of the cofactors, Mg2" and thiamine diphosphate. When flavine adenine dinucleotide was added along with Mg2+ and thiamine diphosphate, an increase in the rate of sedimentation occurred that was thought to be due to a rapid tetramer-dimer interconversion. The addition of pyruvate, the substrate, along with the three cofactors, resulted in a further increase in sedimentation rate, due presumably to an increase in the tetramer-to-dimer ratio. The addition of valine to the complete system resulted in maintenance of the enzyme in the dimeric state concomitant with inhibition of enzyme activity. iWllO (valine resistance) mutations. Finally, acetohydroxy acid synthase III was shown to be the product of a locus near the leu operon which was proposed to contain two genes, ilvH and ilvI. However, owing to the limited evidence for two separate genes, the locus will here be referred to as the iluHI gene. The third isozyme was recently shown by DeFelice and Levinthal (9) to be strongly repressed by leucine alone.A clear understanding of this gene-enzyme relationship will be dependent not only upon a more careful genetic analysis than has been performed until now but also upon a distinction 846 on July 16, 2020 by guest

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