RNA polymerases are a class of enzymes that synthesise RNA transcripts from a DNA template. They bind to ribosome binding sites, and begin transcribing from the ‘transcriptional start site’ (TSS), a start codon.
Sigma factors are essential to allow RNA polymerase to bind to the DNA. When a sigma factor and polymerase are bound, they form a holoenzyme. The formation of a holoenzyme allows the RNA polymerase to recognise, and bind to, promoter sequences.
Different sigma factors can be used to express different regulons (large groups of genes within the chromosome).
In the case of sigma-70, when the polymerase successfully recognises its target promoter sequence, the sigma factor detaches, allowing the formation of an open complex. The polymerase open complex unwinds the DNA, allowing the RNA transcript to be produced. While the sigma factor is bound, the polymerase scans along the DNA template in the closed complex.