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The acetohydroxy-acid synthetases from two extreme bacterial thermophiles,Thermus aquaticus andBacillus sp., have been studied. The two enzymes have different pH optima, 8 and 6, respectively, and both are feedback inhibited by valine. The inhibition is of interest because it is not expressed below 60 C, but only at higher temperatures which are optimal for catalytic activity. Valine inhibition inT. aquaticus was noncompetitive, whereas inBacillus sp., it was competitive. Isoleucine (10−3 M) also inhibited the two enzymes, whereas leucine (10−3 M) did not. There was no concerted feedback when the amino acids were added in together. The sensitivity of the enzymes to valine could not be removed by HgCl2 . Both enzymes required Mg2+ and thiamine pyrophosphate for optimal activity, whereas only the enzyme fromT. aquaticus required flavine adenine dinucleotide in addition. None of these cofactors was essential for the feedback inhibition caused by valine. The enzymes from both bacteria could be repressed, but only in the presence of all three branched-chain amino acids indicating that, as inEscherichia coli andSalmonella typhimurium , the repression system is multivalent.

Comparison of Acetohydroxy-Acid Synthetases from Two Extreme Thermophilic Bacteria