Molecular structure ofilvIHand its evolutionary relationship toilvGinEscherichia coliK12

ilvIH of Escherichia coli K12 codes for a valine-sensitive acetohydroxy acid synthase (AHASIII). The DNA sequence of ilvIH was determined. Open reading frames and appropriate translation signals exist for two polypeptides, one containing 565 amino acids (ilvI polypeptide) and the other 160 amino acids (ilvH polypeptide). A graphic matrix analysis shows three clearcut regions of homology between ilvI and ilvG (codes for AHASII). Within these three regions of homology, 50-60% of the amino acid sequences of AHASII and AHASIII are conserved. Inspection of the region between ilvG and ilvE (the K region) revealed that it can potentially code for an 86 amino acid polypeptide. A computer analysis shows small but significant homology between the predicted amino acid sequences of the N-terminal half of the ilvH polypeptide and the putative region K polypeptide. We conclude that ilvIH and ilvG-region K evolved from a common ancestor.