Characterization of an Extended-Spectrum Class C Beta-Lactamase of Citrobacter Freundii

Microbiol Immunol. 2001;45(4):277-83. doi: 10.1111/j.1348-0421.2001.tb02619.x.


Citrobacter freundii GC3 is a clinical isolate which showed moderate resistance to oxyimino beta-lactams such as ceftazidime and aztreonam. This drug resistance was due to an extended-spectrum class C beta-lactamase encoded by chromosomal gene(s). The GC3 beta-lactamase showed high amino acid sequence homology to a known C. freundii beta-lactamase, i.e., 346 of 361 amino acids were identical with those of C. freundii GN346 beta-lactamase (Tsukamoto, K. et al, Eur. J. Biochem. 188, 15-22, 1990). Asp198 was the only dissimilar amino acid found in the omega loop region, known as the hot spot for extended-spectrum resistance in class C beta-lactamases (Haruta, S. et al, Microbiol. Immunol. 42, 165-169, 1998). However, Asp198 was eliminated as a cause of the extended-spectrum resistance by the substitution of Asn for Asp198. Subsequent investigation suggested that the moderate resistance to oxyimino beta-lactams is attributable to the replacement of amino acids on the enzyme's surface area, far from the active-site. Some or all of the replacements are assumed to delicately modify the active-site configuration. The GC3 beta-lactamase is the first example of an extended-spectrum class C beta-lactamase in which mutations are independent of the omega loop.

MeSH terms

  • Amino Acid Sequence
  • Amino Acid Substitution
  • Base Sequence
  • Catalytic Domain
  • Citrobacter freundii / drug effects
  • Citrobacter freundii / enzymology*
  • Citrobacter freundii / genetics
  • Cloning, Molecular
  • DNA, Bacterial / genetics
  • Drug Resistance, Microbial / genetics
  • Escherichia coli / genetics
  • Gene Expression
  • Genes, Bacterial
  • Humans
  • Models, Molecular
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Protein Conformation
  • beta-Lactamases / chemistry*
  • beta-Lactamases / classification
  • beta-Lactamases / genetics


  • DNA, Bacterial
  • beta-Lactamases