The mobilizable plasmid pMD101 () was constructed by inserting cloned , the structural gene for the chromosomal AmpC β-lactamase of , and the closely linked encoding the transcriptional regulator essential for enzyme induction, into the broad host-range plasmid pKT231. Plasmid pMD101 was transconjugated into VI and its isogenic, cell-wall-less protoplast L-form LVI. AmpC β-lactamase was expressed constitutively from cloned and in bacteria and in some L-form protoplasts. However, induction of the enzyme by β-lactam antibiotics occurred only in bacterial cells and not in the cell-wall- and peptidoglycan-deficient L-form. In agreement with current models, induction of AmpC β-lactamase is thought to be initiated by an induction signal arising from the metabolic disturbance of cell-wall peptidoglycan.


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