The product of the Escherichia coli rpoH (htpR) gene, sigma 32, is required for heat-inducible transcription of the heat shock genes. Previous studies on the role of sigma 32 in growth at low temperature and in gene expression involved the use of nonsense and missense rpoH mutations and have led to ambiguous or conflicting results. To clarify the role of sigma 32 in cell physiology, we have constructed loss-of-function insertion and deletion mutations in rpoH. Strains lacking sigma 32 are extremely temperature sensitive and grow only at temperatures less than or equal to 20 degrees C. There is no transcription from the heat shock promoters preceding the htpG gene or the groESL and dnaKJ operons; however, several heat shock proteins are produced in the mutants. GroEL protein is present in the rpoH null mutants, but its synthesis is not inducible by a shift to high temperature. The low-level synthesis of GroEL results from transcription initiation at a minor sigma 70-controlled promoter for the groE operon. DnaK protein synthesis cannot be detected at low temperature, but can be detected after a shift to 42 degrees C. The mechanism of this heat-inducible synthesis is not known. We conclude that sigma 32 is required for cell growth at temperatures above 20 degrees C and is required for transcription from the heat shock promoters. Several heat shock proteins are synthesized in the absence of sigma 32, indicating that there are additional mechanisms controlling the synthesis of some heat shock proteins.