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Does trp repressor form a tetramer or a dimer before it binds to its corepressor tryptophan? I've been reading BIOS Molecular biology it says that it is a dimer

The repressor is a dimer of two subunits that have structural similarity to the CAP protein and lac repressor.

but according to wikipedia it is tetramer. What is correct?

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Revised answer summary

My original answer provided evidence that the trp repressor is a dimer, and that conclusion stands, especially as regards its functional form. I still regard the statement in Wikipedia that it is a tetramer as incorrect, although I now see how it might have arisen. It seems to be a misinterpretation of results in vitro that show an equilibrium can exist between dimer and tetramer (not tetramer only). I have expanded on this in an additional section.

A functional dimer

The functional form of the is certainly a dimer, as originally reported by Schievitz et al. (1985). The summary of this paper includes the statement:

“The crystal structure of the Escherichia coli trp repressor has been solved to atomic resolution. The dimeric protein has a remarkable subunit interface in which five of each subunit's six helices are interlinked.”

It was crystallized in combination with the operator DNA, and the structure deposited in the Protein Database (PDB) as 1TRO. I reproduce an image from the PDB entry below. Many other DNA-binding proteins also have dimeric structures.

Trp-repressor binding to operator DNA

A dimer–tetramer equilibrium observed in solution

The statement in Wikipedia probably arises from a report by Fernando and Royer (1992) — subsequently confirmed — that in solution there is an equilibrium between the dimer and higher oligomers (the tetramer predominating). Tryptophan was found to shift the equilibrium to the dimer. The authors claim that this has physiological relevance, although this still seems a moot point.

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    $\begingroup$ I have revised my answer, explaining the situation with the tetramer. Wikipedia is incorrect, but it is important to realize that Wikipedia consists entirely of the voluntary work of people like us. It doesn't claim to be correct, only correctable. I feel that it is an obligation of those who find mistakes to correct them (if they can do it easily and with authority) and I shall do so myself in this case. I encourage others to do likewise. $\endgroup$ – David Aug 4 '16 at 13:18
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    $\begingroup$ I have now edited the Wikipedia page. I also deleted a reference to nucleoplasm in this section, which should have been removed long ago. Needless to say, bacteria have no nuclei and no nucleoplasm. $\endgroup$ – David Aug 4 '16 at 14:17
  • $\begingroup$ I saw you editing today. The original article was poorly written. $\endgroup$ – Tyto alba Aug 16 '16 at 12:25
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    $\begingroup$ @SanjuktaGhosh — Unfortunately many of the articles on transcription in WIkipedia are poor or out of date. The main article is terrible, failing to distinguish between eukaryotic and prokaryotic systems. The problem seems to be that when Wikipedia started there was an initial surge in enthusiasm, but the updating has been very uneven across topics. Younger people who try to help are often brought up on eukaryotic molecular biology and are unaware of the older work on what then were the only tractable systems — prokaryotes. Specialists generally have little time or incentive to contribute. $\endgroup$ – David Aug 16 '16 at 14:20

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