Transcription initiation is a key step at which gene expression can be regulated. The sigma subunit of RNA polymerase provides the enzyme with the ability to recognize promoter sequences and initiate transcription at specific sites on the chromosome. The molecular basis of sigma function is not well known. It has been suggested that sigma factors confer promoter specificty by making direct contacts to the promoter DNA (Losick and Pero, 1981). To test this idea, suppressors of promoter down mutations were sought that affected the promoter recogniton properties of the σ⁷⁰ subunit of E. coli RNA polymerase. Four such sigma mutants were obtained, two of which are allele-specific. One of these mutants has a change at a position in the predicted helix-turn-helix DNA binding structure which lies in a conserved region of the protein (region 4). This mutant specifically suppresses promoter down mutations in the -35 region of the promoter. The other mutant has a change at a residue that lies in a predicted α-helix of conserved region 2. This mutant specifically suppresses promoter mutations in the -10 region of the promoter. These data support the idea that regions 2 and 4 of sigma interact with the -10 and -35 regions of the promoter, respectively.