We have determined the nucleofide sequence of the blaT-3 gene of plasmid pCF04 which confers resistance to penicillins and most cephalosporins by mediating the production of TEM-3 fl-lactamasc. The deduced amino acid sequence of TEM~3 differed in two positions from that of the TEM-2 penicillinase: Lys (TEM-3) for Glu (TEM-2) at position 102, and Ser (TEM-3) for Gly (TEM-2) at position 236 of the unprocessed protein. Examination of the location of the two modified amino acids of TEM-3 in the tertiary structure of class A fl-lactamases suggested that they both are part of the substrate binding site. Analysis, by in vitro recombination, indicated that each mutation contributes to the extended substrate range of the enzyme, compared to that of the TEM-type penicillinase, and that the strength of the promoter of
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Sougakoff et al. (1988) studied this question.
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