Imidazole acetol phosphate aminotransferase in Zymomonas mobilis: molecular genetic, biochemical, and evolutionary analyses

J Bacteriol. 1995 Mar;177(6):1576-84. doi: 10.1128/jb.177.6.1576-1584.1995.


hisH encodes imidazole acetol phosphate (IAP) aminotransferase in Zymomonas mobilis and is located immediately upstream of tyrC, a gene which codes for cyclohexadienyl dehydrogenase. A plasmid containing hisH was able to complement an Escherichia coli histidine auxotroph which lacked the homologous aminotransferase. DNA sequencing of hisH revealed an open reading frame of 1,110 bp, encoding a protein of 40,631 Da. The cloned hisH product was purified from E. coli and estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to have a molecular mass of 40,000 Da. Since the native enzyme had a molecular mass of 85,000 Da as determined by gel filtration, the active enzyme species must be a homodimer. The purified enzyme was able to transaminate aromatic amino acids and histidine in addition to histidinol phosphate. The existence of a single protein having broad substrate specificity was consistent with the constant ratio of activities obtained with different substrates following a variety of physical treatments (such as freeze-thaw, temperature inactivation, and manipulation of pyridoxal 5'-phosphate content). The purified enzyme did not require addition of pyridoxal 5'-phosphate, but dependence upon this cofactor was demonstrated following resolution of the enzyme and cofactor by hydroxylamine treatment. Kinetic data showed the classic ping-pong mechanism expected for aminotransferases. Km values of 0.17, 3.39, and 43.48 mM for histidinol phosphate, tyrosine, and phenylalanine were obtained. The gene structure around hisH-tyrC suggested an operon organization. The hisH-tyrC cluster in Z. mobilis is reminiscent of the hisH-tyrA component of a complex operon in Bacillus subtilis, which includes the tryptophan operon and aroE. Multiple alignment of all aminotransferase sequences available in the database showed that within the class I superfamily of aminotransferases, IAP aminotransferases (family I beta) are closer to the I gamma family (e.g., rat tyrosine aminotransferase) than to the I alpha family (e.g., rat aspartate aminotransferase or E. coli AspC). Signature motifs which distinguish the IAP aminotransferase family were identified in the region of the active-site lysine and in the region of the interdomain interface.

Publication types

  • Comparative Study

MeSH terms

  • Amino Acid Sequence
  • Amino Acids / metabolism
  • Base Sequence
  • Biological Evolution
  • Cloning, Molecular
  • Escherichia coli / genetics
  • Genes, Bacterial / genetics*
  • Genetic Linkage
  • Molecular Sequence Data
  • Multigene Family
  • Pyridoxal Phosphate / metabolism
  • RNA, Messenger / genetics
  • Restriction Mapping
  • Sequence Analysis
  • Sequence Homology, Amino Acid
  • Substrate Specificity
  • Transaminases / chemistry
  • Transaminases / classification
  • Transaminases / genetics*
  • Transaminases / isolation & purification
  • Transaminases / metabolism
  • Tryptophan / biosynthesis
  • Zymomonas / enzymology
  • Zymomonas / genetics*


  • Amino Acids
  • RNA, Messenger
  • Pyridoxal Phosphate
  • Tryptophan
  • Transaminases
  • histidinol-phosphate aminotransferase

Associated data

  • GENBANK/L36343