The sigma subunit of prokaryotic RNA polymerase:
Prokaryotic RNA polymerase:

In bacteria, one species of RNA pol synthesizes all of the RNA except for the short RNA primers needed for DNA replication RNA pol is a multisubunit enzyme that recognizes a nucleotide sequence (the promoter region) at the beginning of a length of DNA that is to be transcribed. 
It next makes a complementary RNA copy of the DNA template strand and then recognizes the end of the DNA sequence to be transcribed (the termination region). 
RNA is synthesized from its 5′-end to its 3′-end, antiparallel to its DNA template strand . 
The template is copied as it is in DNA synthesis, in which a guanine (G) on the DNA specifies a cytosine (C) in the RNA, a C specifies a G, a thymine (T) specifies an adenine (A), but an A specifies a uracil (U) instead of a T 
The RNA, then, is complementary to the DNA template (antisense, minus) strand and identical to the coding (sense, plus) strand, with U replacing T. 
Within the DNA molecule, regions of both strands can serve as templates for transcription. For a given gene, however, only one of the two DNA strands can be the template. 
Which strand is used is determined by the location of the promoter for that gene. 
Transcription by RNA pol involves a core enzyme and several auxiliary proteins.

Multisubunit Enzyme:
Core Enzyme + σ subunit = Holoenzyme (E σ).
Core Enzyme consist 2 α and 1β and 1 β’ and ω subunit.
Sigma subunit help RNA polymerase to bind to the promoter site.
Beta subunit is the catalytic subunit.  β subunit binds the Mg 2+ ions.
Ref : Lippincott’s illustrated reviews for biochemistry , 7th edition.
Binds the antibiotic rifampicin
Is inhibited by a-amanitin
Specifically recognizes the promoter site
Is part of the core enzyme