Nucleotide sequences of the genes coding for the TEM‐like β‐lactamases IRT‐1 and IRT‐2 (formerly called TRI‐1 and TRI‐2)

Two bla TEM ‐like genes were characterized that encoded IRT β‐lactamases (previously called TRI) in clinical isolates of Escherichia coli resistant to amoxycillin alone and to combinations of amoxycillin with β‐lactamase inhibitors. Plasmids carrying this resistance were isolated from E. coli K 12 transconjugants and the genes were sequenced after amplification of defined fragments, using TEM‐1‐specific primers. The gene for IRT‐1 β‐lactamase resembled the bla TEM‐1B gene, and that for IRT‐2 resembled bla TEM‐2 . However, both IRT enzymes have a glutamine residue at position 37, which is characteristic of TEM‐1. The unique nucleotide difference with parental genes corresponding to amino acid variation was observed at nucleotide position 929. The consequence of C to T transition in the bla IRT‐1 gene and C to A transversion in the bla IRT‐2 gene was the substitution of arginine 241 in the native protein by cysteine and serine, respectively, in the mutants. Thus, the nature of amino acid 241 is critical in conferring resistance or susceptibility to β‐lactamase inhibitors. Furthermore, these basic to neutral amino acid replacements explain the more acidic p I (p I =5.2) of these IRT enzymes compared to that of TEM‐1 (p I =5.4). The presence of cysteine‐241 in IRT‐1 also explains the selective sensitivity of this β‐lactamase to inhibition by p ‐chloromercuribenzoate.