The expression of the ibpAibpB heat-shock operon of Escherichia coli was found previously not to conform to the known pattern of expression of the sigma (32)-regulated operons because the rpoH gene mutation inactivating the sigma (32) protein did not abolish the ibp induction. We show here that this effect can depend partly on the sigma (54)-promoter that is inducible by heat shock, located upstream of the ibpB, the distal gene of the operon. It may also depend on a metabolic signal, postulated by others, and possibly required for the expression of the ibpAB genes. Thus, the ibpB gene can be translated from the transcript covering the whole operon starting from the sigma (32)-promoter and from the ibpB gene transcript starting from the sigma (54)-promoter. These results indicate that the ibpB gene is a second member of the sigma (54)-heat-shock regulon in E. coli besides pspA-E operon. Thus, heat-shock response involves three regulons controlled by sigma (32), sigma (24), and sigma (54) RNA polymerase subunits.