The gene for CspA, the major cold-shock protein of Escherichia coli is known to be dramatically induced upon temperature downshift. Here, we report that three-base substitutions around the Shine-Dalgarno sequence in the 159-base 5'-untranslated region of the cspA mRNA stabilizes the mRNA 150-fold, resulting in constitutive expression of cspA at 37 degrees C. This stabilization was found to be at least partially due to resistance against RNase E degradation. The cold-shock induction of cspA was also achieved by exchanging its promoter with the non-cold-shock Ipp promoter. The results presented indicate that the cspA gene is efficiently transcribed even at 37 degrees C. However, the translation of the cspA mRNA is blocked because of its extreme instability at 37 degrees C. The presented results also demonstrate that the cspA gene is constitutively transcribed at all temperatures; however, its expression at 37 degrees C is prevented by destabilizing its mRNA.