RNA polymerase sigma-K type <p>The bacterial core RNA polymerase complex, which consists of five subunits, is sufficient for transcription elongation and termination but is unable to initiate transcription. Transcription initiation from promoter elements requires a sixth, dissociable subunit called a sigma factor, which reversibly associates with the core RNA polymerase complex to form a holoenzyme [<cite idref="PUB00000061"/>]. RNA polymerase recruits alternative sigma factors as a means of switching on specific regulons. Most bacteria express a multiplicity of sigma factors. Two of these factors, sigma-70 (gene rpoD), generally known as the major or primary sigma factor, and sigma-54 (gene rpoN or ntrA) direct the transcription of a wide variety of genes. The other sigma factors, known as alternative sigma factors, are required for the transcription of specific subsets of genes.</p> <p> With regard to sequence similarity, sigma factors can be grouped into two classes, the sigma-54 and sigma-70 families. Sequence alignments of the sigma70 family members reveal four conserved regions that can be further divided into subregions eg. sub-region 2.2, which may be involved in the binding of the sigma factor to the core RNA polymerase; and sub-region 4.2, which seems to harbor a DNA-binding 'helix-turn-helix' motif involved in binding the conserved -35 region of promoters recognised by the major sigma factors [<cite idref="PUB00004340"/>, <cite idref="PUB00002181"/>]. </p><p>The sporulation-specific transcription factor sigma-K (also called sigma-27) is expressed in the mother cell compartment of endospore-forming bacteria such as <taxon tax_id="1423">Bacillus subtilis</taxon>. Like its close homologue sigma-E (sigma-29, see <db_xref db="INTERPRO" dbkey="IPR014200"/>), also specific to the mother cell compartment, it must be activated by a proteolytic cleavage.</p> <p>Note: that in B. subtilis (and apparently also in <taxon tax_id="1513">Clostridium tetani</taxon>), but not in other endospore forming species such as <taxon tax_id="1392">Bacillus anthracis</taxon>, the sigK gene is generated by a non-germline (mother cell only) chromosomal rearrangement that recombines coding regions for the N-terminal and C-terminal regions of sigma-K [<cite idref="PUB00034451"/>].</p>