Fig. 1.

β-Lactamase expression in the Alg⁺ PDOampR mutant. Assays were performed using the parent strain Alg⁺ PDO300, the mutant Alg⁺ PDOampR and Alg⁺ PDOampR (pAmpR) in the absence (−) and presence (+) of an inducer. The plasmid pAmpR carries the wild-type ampR gene on a broad-host-range low-copy-number plasmid pME6030 (Heeb et al., 2000). Fresh cultures of OD₆₀₀ 0.6–0.8 were induced with 100 μg benzylpenicillin ml⁻¹ for 3 h before harvesting. Assays were performed on sonicated lysate using nitrocefin as a chromogenic substrate. Assays were performed in triplicate. One Miller unit of β-lactamase is defined as 1 nmol nitrocefin hydrolysed min⁻¹ (μg protein)⁻¹.