Post-transcriptional control of the cyanobacterial hspA heat-shock induction

The hspA gene encodes a small heat-shock protein in the thermophilic cyanobacterium Synechococcus vulcanus. To gain insight into the post-transcriptional regulation, hspA was expressed in Escherichia coli. HspA was induced upon a temperature upshift from 30 to 42 degrees C, although the hspA transcription in E. coli occurred constitutively at both 30 and 42 degrees C. Neither replacement of the native hspA promoter with the lacZ promoter nor the addition of rifampin abolished the heat induction. Thus, the primary form of regulation of the heat induction is at the post-transcriptional level. Analyses of expression of a series of the transcriptional and translational hspA-lacZ fusions confirmed the constitutive transcription, and demonstrated that the heat induction occurred only in the translational fusions. They further indicated the presence of regulatory elements involved in the translational regulation. An element in the 5'-untranslated region of the hspA mRNA suppressed the translation, while that in the hspA coding region was required for the de-repression of the translation and for thermal regulation.