The katF gene of Escherichia coli has been sequenced revealing a 1086 base pair open reading frame from which the sequence of a 362 amino acid protein has been deduced. The direction of transcription of katF was confirmed by expression of the gene cloned in both directions behind a T7 promoter. The KatF protein expressed in vitro migrates with an apparent size of 42 kDa. Comparison of the katF sequence to the sequence of rpoD, which encodes the sigma subunit of RNA polymerase, revealed a 181 bp region with 65% homology and a 38 bp segment that was 87% homologous. A 62 amino acid region of the predicted KatF protein sequence was found to be 85% homologous to the corresponding sequence of sigma 70, including a segment implicated in core polymerase binding. Homology was also observed with the heat shock regulatory protein encoded by htpR.