Sequence of the Citrobacter freundii OS60 chromosomal ampC beta-lactamase gene

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Abstract
The Citrobacter freundii OS60 ampC .beta.-lactamase gene was sequenced and found to encode a 380-amino-acid-long precursor with a 19-residue signal peptide. The mature protein has a predicted molecular mass of 39781 Da. The first 60 residues of the purified enzyme, as determined by sequential Edman degradation, are identical to the amino acid sequence inferred from the gene sequence. Also, the amino acid composition determined for the purified .beta.-lactamase and that given by the gene sequence are in good agreement. 77% of the amino acid positions hold identical residues in the C. freundii and Escherichia coli K12 chromosomal AmpC .beta.-lactamases. This clearly puts the C. freundii enzyme into the class C of .beta.-lactamases. Of the 68 amino-terminal residues determined for the Enterobacter cloacae P99 .beta.-lactamase, 44 are identical to the corresponding residues of the C. freundii enzyme. All three enzymes, as well as that of Pseudomonas aeruginosa 18S/H are highly similar around the active-site serine at position 64 of the mature protein.