Promoters and initiation:

In E. coli whole genes are transcribed through a single huge RNA polymerase with the subunit structure α2 ββ. This whole  enzyme, known the holoenzyme, is needed to initiate transcription  because the factor is essential for recognition of the promoter; it decreases  the affinity of the core enzyme  for nonspecific  DNA connecting sites and rise its affinity for the  promoter. It is general for prokaryotes to have various factors which recognize different kinds of promoter. The holoenzyme binds to a promoter region about 40-60 bp in size and then initiates transcription a small distance downstream example for 3 to the promoter. Within the promoter lie two 6-bp series  that are particularly  important  for promoter function and that  are thus  highly conserved among  species. By Using the convention  of calling the first nucleotide  of a transcribed sequence as 1, these two promoter components  lie at positions  -10 and -35, that is about 10 and 35 bp, in that order, upstream of where transcription will start.

- The -10 series has the consensus TATAAT.  Because this part was discovered through Pribnow, it is also called as the Pribnow box. It is an must recognition site which interacts with the σ factor of RNA polymerase.

- The  -35  sequence  has  the  consensus TTGACA and is must in DNA unwinding in during transcriptional  initiation.

The real sequence among the -10 sequence and the -35 sequence is not conserved for instance it varies from promoter to promoter  but the distance  among these two sites is extremely important for right functioning of the promoter.

Promoters differ through up to 1000-fold  in their  efficiency  of initiation  of transcription  so which  genes  with  strong  promoters  are transcribed  extremely  frequently

figure: Prokaryotic promoter showing the -10 sequence and -35 sequence. By convention, the first nucleotide of the template DNA  that is transcribed into RNA is denoted   1, the transcriptional start site.

as where genes with weak promoters are transcribed far less frequently. The -10,-35 sequences of strong promoters correspond well with the consensus sequences shown in Figure although weaker promoters should have sequences which differ from these at one or more nucleotides. The nature of the sequences across the transcriptional begins site cans also influence the efficiency of initiation.  The RNA polymerase does not require a primer to start transcription cf.  DNA polymerases that will having bound to the promoter site the RNA polymerase starts transcription directly.