The operon that encodes the sigma subunit of RNA polymerase also encodes ribosomal protein S21 and DNA primase in E. coli K12.

The sigma subunit of E. coli RNA polymerase is encoded by the rpoD gene. Within the sequence upstream from rpoD, we have identified the structural genes rpsU and dnaG, which encode the 30S ribosomal protein S21 and DNA primase, respectively. The three genes are in the order rpsU, dnaG rpoD, and are all encoded by the same DNA strand. Analysis of in vivo transcripts from this region shows that these genes are all within the same operon. By correlating the 5' and 3' ends of in vivo transcripts with our DNA sequence, we have identified several regulatory features of the operon. These features include tandem promoters upstream from rpsU, a terminator between rpsU and dnaG, an RNA processing site separating dnaG and rpoD, and the operon terminator just downstream from rpoD. Immediately upstream of the operon promoters is an active promoter for an unidentified gene. We discuss the regulatory significance of the operon features and the biological significance of an operon encoding proteins essential for translation, replication and transcription.