The aggregation-prone fusion protein preS2–S′–β-galactosidase was used as a model system to compare the efficiencies of the IPTG-inducibletacpromoter and the low-temperature-induciblecspApromoter in directing the expression of soluble recombinant polypeptides at reduced growth temperatures inEscherichia coli.At 37°C, the fusion protein was produced at high levels from thetacpromoter, but aggregated quantitatively in a biologically inactive form. In contrast, little preS2–S′–β-galactosidase was synthesized from thecspApromoter at this temperature, presumably due to transcript instability. The highest yields of active enzyme were obtained following temperature downshift from 37 to 30°C for thetacpromoter and 25°C for thecspApromoter. At 25°C, the kinetics of accumulation of β-galactosidase activity, ratios of soluble to insoluble fusion protein, and synthesis rates of preS2–S′–β-galactosidase were virtually identical for both promoters for a period of 2 h postinduction. Thereafter, thecspApromoter became repressed, whereas synthesis of the fusion protein continued with thetacsystem. Following transfer to 10°C, thetacpromoter was almost completely inhibited while thecspApromoter was able to direct the synthesis of soluble preS2–S′–β-galactosidase for up to 2 h. However, the levels of active enzyme produced were approximately threefold lower than those measured at 25°C. Overexpression of native CspA had no effect on the accumulation of active preS2–S′–β-galactosidase from thecspApromoter. It is therefore unlikely that CspA acts as it own positive inducer. Our results indicate that thecspApromoter can efficiently substitute for thetacsystem at 25°C and may be particularly valuable for the expression of highly aggregation-prone or unstable gene products at 10°C.