The GlnAP2 element has been proved to be an effective and inducible - by exogenous acetate - promoter in Escherichia coli with glnL/pta double mutations. Based on this feature, a single-copy expression vector was constructed via coupling of the glnAP2 promoter-regulated T7 RNA polymerase gene and the T7-promoter-controlled lacZ gene on a bacterial artificial chromosome. After induction with 20 mM potassium acetate, the glnL/ pta double mutant E. coli harboring the single-copy plasmid produced 47,500 Miller units of beta-galactosidase activity. This high level expression, corresponding to 27% of total cell protein, was comparable to that determined with the commercial multi-copy expression vector, pET-14b, in strain E. coli Tuner (DE3) (64,300 Miller units, 41% of total cell protein). Moreover, this single-copy expression vector could be maintained for at least 150 generations even in the presence of inducers. In contrast, the multi-copy expression vector was extensively lost after induction. The results indicate that the single-copy expression system has the potential for high-level heterologous protein production for industrial applications.