Escherichia coli K-12 produces at least two ATP-dependent proteases, Lon (La) and Clp (Ti), the latter consisting of a regulatory subunit (ClpA) and a proteolytic subunit (ClpP). The gene clpB encoding an analog of ClpA had been found at 57 min on the E. coli chromosome. Cloning and examination of novel heat shock promoters led us to identify a major clpB promoter specifically controlled by a heat shock sigma factor, sigma 32 (the rpoH [= htpR] gene product). beta-Galactosidase synthesis from a PclpB-lacZ operon fusion was transiently induced upon temperature shift from 30 to 42 degrees C, and the induction depended on the rpoH function. Chromosomal clpB transcripts also increased upon temperature upshift and were totally absent in the rpoH deletion strain. In the in vitro transcription experiments, the clpB promoter was specifically recognized and transcribed by RNA polymerase-sigma 32. Nucleotide sequencing and determination of mRNA start sites permitted us to identify a major heat shock promoter located upstream of the clpB coding sequence. The results clearly indicate that clpB expression is under direct control of sigma 32. Since ClpP was recently shown to be a sigma 32-dependent heat shock protein, the present finding suggests the possibility that a potential ATP-dependent protease, ClpB-ClpP complex, plays an important role against thermal stress in E. coli.