The enzyme acetohydroxy acid synthase (AHS), which catalyses the first common step in the biosynthesis of isoleucine, leucine and valine, has been demonstrated to be present in in two isoenzymic forms. The complete nucleotide sequences of the genes and encoding these two enzymes have been determined. Sequence analysis revealed the presence of two open reading frames, of 1836 and 1737 nucleotides for and , respectively. The predicted amino acid sequences of the two isoenzymes, compared with the PCC 7942 AHS enzyme and the large subunits of the AHSI, II, III isoenzymes, revealed a notable degree of similarity. A small subunit has not been identified for either of the AHS isoenzymes. Analysis by Northern blot hybridization demonstrated that the and genes are transcribed to give mRNA species of approximately 2.15 kb and 1.95 kb, respectively.


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