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

  • 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.

  • 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?

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    The promoters are usually on the coding strand (which is the one which is transcribed) in front of the transcription start site. – Chris Jan 06 '14 at 20:07
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    @Chris I don't think it is correct to say that the coding strand is transcribed - the coding strand of the DNA has the same sequence as the RNA produced by transcription (except T>U). The other strand of the DNA is the template for the synthesis of the RNA. – Alan Boyd Jan 06 '14 at 22:14
  • You are right. I still get confused by this every now and then. Thanks for correcting. – Chris Jan 06 '14 at 22:28
  • It occurs to me that there is a usage of "or" that can confuse. Above "are on the coding or template strand" might as well be written "are on the coding strand un-commonly also known as the template strand". However, clearly the author wants to know on which side promoters are on, physically. – Peter Bernhard Apr 02 '22 at 18:45
  • Refering to "...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..." There lies no contraction; why do you say "and then" (sounds German to me:-). In other words, promoter may lie on the one side, for the one side there is graphics given. That seem to be coherent, from the side of the textbook. – Peter Bernhard Apr 02 '22 at 18:51
  • Anyway, can you offer some link or say what textbook you were reading? Any other? Me, I was surprised to find out that promoter "sits", see below, in the template/anti-sense strand, as I learnt, before, that is may sit on the coding strand, Thanks to your question I now may assume it's different for prokaryotes. Do you know of any parallel rule/exception issue with eukaryotes? – Peter Bernhard Apr 02 '22 at 18:55

2 Answers2

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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.

Alan Boyd
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  • "The next step is the switch to the open complex" - difficult to imagine that polymerase does not need the strands already open to bind.
  • From the abstract of the link you gave, sigma seems specific to species. One sigma for each species. Is is relevant to the question?
  • "...the promoter property of the sequence is due to ... and its complement on the other strand." By "property" you mean that - as both strands have stayed together - the promoter might as well be defined by the sense strand, which is called the coding strand, may I say "denominating strand"?
    • – Peter Bernhard Apr 02 '22 at 18:39
  • Again, according to your answer, at least with eukaryotes, the promoter physically never lies or acts in the coding/sense strand, right? (This is so essential...) – Peter Bernhard Apr 02 '22 at 18:58