The effects on cloned amylase production of co-overexpression of prlF, a gene that appears to interact with the sec protein export machinery in Escherichia coli, was investigated by comparing three expression systems : (i) a high copy number plasmid with the Bacillus stearothermophilus alpha-amylase gene (amyS) cloned with its promoter downstream of the lac promoter; (ii) a pBR322-based vector with amyS under control of the indigenous Bacillus promoter; and (iii) a temperature-inducible vector with runaway replicon and lambda p(L) promoter-controlled gene expression. In addition, protease mutants (lon(-)) of E. coli C600 were used to evaluate the influence of the Lon protease on net enzyme formation and activity degradation during batch fermentations. Our results show that alpha-amylase synthesis occurred during exponential growth and ceased in the stationary phase. While strong promoters on high copy number plasmids severely impaired cell viability, resulting in culture lysis at mid-log phase, co-overexpression of prlF greatly improved cell viability, as well as the yield and specific production of alpha-amylase for the expression constructs considered. ion deficiency slightly increased amylase stability during the late stationary phase. However, the specific productivity of lon(-) strains was only about 40-60% that of the isogenic E. coli C600 equivalent.