Transcription of the ibpB heat-shock gene is under control of sigma(32)- and sigma(54)-promoters, a third regulon of heat-shock response

The expression of the ibpAibpB heat-shock operon of Escherichia coli was fo und previously not to conform to the known pattern of expression of the sig ma (32)-regulated operons because the rpoH gene mutation inactivating the s igma (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 h eat 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 r equired for the expression of the ibpAB genes. Thus, the ibpB gene can be t ranslated from the transcript covering the whole operon starting from the s igma (32)-promoter and from the ibpB gene transcript starting from the sigm a (54)-promoter. These results indicate that the ibpB gene is a second memb er of the sigma (54)-heat-shock regulon in E. coli besides pspA-E operon. T hus, heat-shock response involves three regulons controlled by sigma (32), sigma (24), and sigma (54) RNA polymerase subunits. (C) 2001 Academic Press .