The sigma (σ) subunit of prokaryotic RNA polymerase
Sigma (σ) factor recognizes promotor sequences on the DNA template. Different factors recognize a different group of genes.
α-amanitin inhibit RNA polymerase II of eukaryotes.
Rifampicin inhibits β-subunit of RNA polymerase.
Sigma unit is not a part of core enzyme; it is a part of holoenzyme.

RNA polymerase (holoenzyme) = core enzyme (2α, β,( β’) + sigma (σ) subunit
Binds the antibiotic rifampicin
Is inhibited by α-amanitin
Specifically recognizes the promoter site
Is part of the core enzyme