Bacterial heat-shock response is a global regulatory system required for effective adaptation to changes (stress) in the environment. Several of the important genes involved in this control, such as the genes coding for the chaperones GroE and DnaK (the bacterial homologues of Hsp60 and Hsp70) are localized in operons, with organization typical of the phylogenetic group. In Escherichia coli, where it has been studied initially, the expression of the heat-shock operon is transcriptionally controlled by the employment of the heat- shock transcription activator - factor σ32, that recognizes specific heat-shock promoters. Later studies indicated that in most bacteria the control of these heat-shock operons is more complex than in the γ-purple proteobacteria and involves several regulatory elements. One such control element is a repressor that regulates transcription of heat-shock genes by binding to a conserved regulatory inverted repeat (IR=CIRCE) located upstream to heat- shock operons. In addition, this IR determines the stability of the transcript, thus controlling the level of translation. Sequence analyses suggest that the IR-dependent control of heat- shock genes was the first control element and was lost during evolution in several phylogenetic groups, such as the γ-purple proteobacteria.