Multiple layers of control govern expression of the Escherichia coli ibpAB heat-shock operon

The Escherichia coli ibpAB operon encodes two small heat-shock proteins, the inclusion-body-binding proteins IbpA and IbpB. Here, we report that expression of ibpAB is a complex process involving at least four different layers of control, namely transcriptional control, RNA processing, translation control and protein stability. As a typical member of the heat-shock regulon, transcription of the ibpAB operon is controlled by the alternative sigma factor σ(32) (RpoH). Heat-induced transcription of the bicistronic operon is followed by RNase E-mediated processing events, resulting in monocistronic ibpA and ibpB transcripts and short 3'-terminal ibpB fragments. Translation of ibpA is controlled by an RNA thermometer in its 5' untranslated region, forming a secondary structure that blocks entry of the ribosome at low temperatures. A similar structure upstream of ibpB is functional in vitro but not in vivo, suggesting downregulation of ibpB expression in the presence of IbpA. The recently reported degradation of IbpA and IbpB by the Lon protease and differential regulation of IbpA and IbpB levels in E. coli are discussed.