Molecular characterization of the genes encoding acetohydroxy acid synthase in the cyanobacterium Spirufina platensis

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 Spirufinupfutensis in two isoenzymic forms. The complete nucleotide sequences of the genes ifuX and ifu W encoding these two enzymes have been determined. Sequence analysis revealed the presence of two open reading frames, of 1836 and 1737 nucleotides for ifuX and ifu W, respectively. The predicted amino acid sequences of the two isoenzymes, compared with the Synectiococcus PCC 7942 AHS enzyme and the large subunits of the Escherichiu coli AHSI, 11, I11 isoenzymes, revealed a notable degree of similarity. A small subunit has not been identified for either of the S. platemis AHS isoenzymes. Analysis by Northern blot hybridization demonstrated that the ifuX and i fa W genes are transcribed to give mRNA species of approximately 2.15 kb and 1-95 kb, respectively.