B) Prokaryotic promoter: Promoter typically consists of 40 bp region located near to 5’ end side of transcription start site. Promoter region consists of two 6 bp consensus sequences elements- Pribnow box or TATA box and -35 region. Pribnow box­ 10 bp upstream of start point is a consensus sequence TATAAT, -35 region has consensus sequence TTGACA.  Structure of Prokaryotic promoter is given  in Figure 28.4.

Figure 28.4 : Structure of Prokaryotic Promoter

TRANSCRIPITION OCCURS IN FOUR STAGES: 1) Template binding, 2) Chain initiation, 3) Chain elongation, 4) Termination.

1)  Binding of RNA polymerases to template DNA and Chain initiation: DNA duplex should be opened so that RNA pol can approach to single stranded DNA template. Efficiency of initiation is inversely proportional to melting temperature Tm, and AT rich region has lower Tm because of double hydrogen bond stabilizing them than triple bond in GC rich region and thus is more stable. Therefore, AT rich region is good for melting of duplex and easy to create open promoter complex than GC rich region. RNA polymerase  has ϭ  factor which recognize promoter sequence at which RNA polymerase Holoenzyme binds and form a complex which known as closed complex. In fact, sigma factor is released when chain reaches nearly up to 10 bases, leaving core enzyme for further elongation, which are given below in Figure 28.5.

Figure 28.5 : Initiation of Transcription