By using a PCR approach, the Bradyrhizobium japonicum sigA gene, which encodes the primary RNA polymerase sigma factor, sigma80, was cloned and its nucleotide sequence was established. The deduced protein is highly homologous to the SigA protein of Rhizobium meliloti (72% amino acid sequence identity) but less so to RpoD of Escherichia coli (51% identity). Well conserved is the C-terminal end of the protein, which is probably involved in promoter recognition and binding of the RNA polymerase core enzyme. A remarkable feature of the primary sequence is an alanine- and proline-rich segment of 24 amino acids between conserved regions 1 and 2, which might function as an interdomain linker. We purified the B. japonicum RNA polymerase holoenzyme. One of the subunits had an apparent molecular mass of 90 kDa and corresponded to the sigA gene product, as judged by N-terminal amino acid sequencing. The purified RNA polymerase was used in an in vitro transcription system to determine the transcription start sites of the rrn and groESL4 operons. They were identical to those previously identified in vivo. The rrn promoter was cloned upstream of a rho-independent terminator, yielding a transcript of about 240 bases. This served as a suitable template to analyze promoter activity. Then mutant derivatives of the rrn promoter were constructed and tested in in vitro transcription experiments. Several base pairs essential for promoter activity were thus identified. The results suggest that the well-characterized -35/-10 promoter class is predominantly used in B. japonicum for the expression of "housekeeping" genes.