The rpoD1 gene of Synechococcus sp. strain PCC 7942 encodes the principal sigma factor of RNA polymerase

RNA polymerase was purified from the unicellular cyanobacterium, Synechococcus sp. strain PCC 7942, and found to be associated with a 52 kilodalton (kDa) polypeptide. The determined N-terminal sequence of the polypeptide was identical to the predicted amino-acid sequence of the rpoD1 gene product. Furthermore, the rpoD1 gene is suggested to be indispensable for viability by the inability to disrupt the gene. These results indicate that the rpoD1 gene product is the principal sigma factor of RNA polymerase.     

Principal sigma subunit of the Caulobacter crescentus RNA polymerase.

We have identified the gene encoding the Caulobacter crescentus principal sigma subunit, RpoD. The rpoD gene codes for a polypeptide of 653 amino acids with a predicted molecular mass of 72,623 Da (sigma 73). The C. crescentus sigma subunit has extensive amino acid sequence homology with the principal sigma factors of a number of divergent procaryotes. In particular, the segments designated region 2 that are involved in core polymerase binding and promoter recognition were identical among these bacteria despite the fact that the -10 region recognized by the C. crescentus sigma 73 differs significantly from that of the other bacteria. Thus, it appears that additional sigma factor regions must be involved in -10 region recognition. This conclusion was strengthened by a heterologous complementation assay in which C. crescentus sigma 73 was capable of complementing the Escherichia coli rpoD285 temperature-sensitive mutant. Furthermore, C. crescentus sigma 73 conferred new specificity on the E. coli RNA polymerase, allowing the expression of C. crescentus promoters in E. coli. Thus, the C. crescentus sigma 73 appears to have a broader specificity than does the sigma 70 of the enteric bacteria.     

Bacillus subtilis dnaE encodes a protein homologous to DNA primase of Escherichia coli

Bacillus subtilis dnaE encodes a protein essential for DNA replication and is tightly linked to rpoD, the gene for the major sigma factor of RNA polymerase. We have now determined the 1809-base pair sequence of the dnaE coding region, which precedes rpoD and is transcribed in the same counterclockwise direction on the chromosome. From the DNA sequence, we found that the dnaE protein comprised 603 amino acids with a calculated molecular mass of 68,428 daltons. This protein had significant and extensive regions of homology with Escherichia coli DNA primase, the polymerase that synthesizes short RNA primers during discontinuous DNA replication. Features of the coding and flanking regions that may modulate dnaE expression include a relatively weak ribosomal binding site (delta G' = -13.8 kcal), the use of uncommon codons in the reading frame, and no obvious promoter sequence for either dnaE or rpoD. Together, these results suggest that dnaE codes for B. subtilis DNA primase and, in light of the similarities to the organization of the E. coli sigma operon, that expression of dnaE may be coregulated with rpoD in B. subtilis.