molecular biology - On which strand does the promoter sit?

My book keeps giving different indicators as to whether the promoters are on the coding or template strand.

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  It says the -35 region in prokaryotes must be on the coding strand. It also mentions, that the -10 and -35 regions are binding sites for RNA polymerase.

  
  


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  It also says that CAAT and GC, unlike -35 in prokaryotes, can also be on the template strand. Which implies that they are still most common on the coding strand. And then it shows me a figure, where a promoter sequence is shown in the template strand, with the transcription complex attached.

  
  

All the sequences are written 5'-TATAAA-3' (for example). This also implies they are on the coding strand, since the template strand will then be 3'->5' which is the direction that transcription happens in.

Obviously I'm confused, and hope someone can clarify this. In which strand can I find the promoters? Does stuff act on the promoter sequence itself, or its complementary sequence? If the promoter can also be on the other strand, should it be reversed?

Answer

The answer to this question depends upon the definition of the word 'promoter'.

In the simplest possible model of prokaryotic transcription the promoter is the site where RNA polymerase binds to the DNA before initiating RNA synthesis. In this process the σ factor recognises the core promoter elements directing the polymerase to bind to the DNA to form the closed complex. The next step is the switch to the open complex involving separation of the DNA strands.

Articles describing investigating the interaction of the σ factor with the DNA (e.g. here) refer to the protein making contacts with base pairs. I conclude therefore that the original question doesn't make sense - a promoter is a dsDNA entity even though we might describe it in terms of the sequence on one or other of those strands. So, for example, in a promoter the consensus -35 sequence - 5'-TTGACA - would be present on the coding strand (upstream of the coding sequence), but the promoter property of the sequence is due to the presence of this sequence and its complement on the other strand.