Zymomonas mobilis ZM4 is resistant to beta-lactam antibiotics but there are no reports of a beta-lactam resistance gene and its regulation. A putative beta-lactamase gene sequence (ZMO0103) in the genome of Z. mobilis showed a 55% amino acid sequence identity with class C beta-lactamase genes. qPCR analysis of the beta-lactamase transcript indicated a higher level expression of the beta-lactamase compared to the relative transcript quantities in antibiotic-susceptible bacteria. The putative beta-lactamase gene was cloned, expressed in Escherichia coli BL21 and the product, AmpC, was purified to homogeneity. Its optimal activity was at pH 6 and 30A degrees C. Further, the beta-lactamase had a higher affinity towards penicillins than cephalosporin antibiotics.