Help! Questions about transcriptional elements in SV40 'control' region

[Sean Stevens]

aurelia at u.washington.edu (A. M. Lotto) wrote:
>Hello,
>	I am in desperate need of an explanation of some transcriptional
>elements of SV40.
>
><-- early   tata       21 bp repeats            72 bp segments
>____________________________________________________________________________
>   |        XXXX  |  21  |  21  | 21  | |     72       |     72       |
>----------------------------------------------------------------------------
>      Ori                                                       late -->
>
>
>My questions regard the sequence of events.  Enhancers bind to TFIID.
>SP1 binds to GC boxes in 21 bp repeat sequences.  TFIID interacts with
>SP1 and SP1 interacts with RNA Pol II which doesn't know where to bind
>the DNA.  First, I am not entirely sure the previous sequence of events
>is even close to correct, secondly, how do these components interact to
>increase the efficiency of txn?  Where does TFIID bind?  My sources say
>that the enhancers bind to the 72 bp sequences but also that TFIID binds
>enhancers.   I am confused about this and would greatly appreciate anyone
>able to shed some light on these interactions.
>
>Thanks very much.
>
>Amy Lotto
>aurelia at u.washington.edu
>

Dear Amy-

Our lab is very much concerned with the above issues, and I think I can clear up some of the misconceptions. First, an enhancer is a DNA sequence, not a protein, and acts positively or negatively to regulate the activity of the promoter of a gene. The promoter is a DNA sequence located around the start of transcription (it includes the start site) and serves as the landing pad for the RNA polymerase. In both cases, the enhancer and the promoter, there are some or many binding sites for proteins called transcription factors, such as Sp1, which collectively influence the rate of transcription, presumably by increasing the number of polymerases brought to a given promoter, or increasing their intrinsic activity. There are also negatively acting transcription factors that serve to decrease the same.

What are the transcription factors talking to? The polymerase does not arrive at the promoter alone-it absolutely requires auxiliary general transcription factors. In the case of RNA polymerase II this includes; TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH and perhaps TFIIG and TFIIJ. These general factors are brought to the promoter in a specific order and only when the complex is assembled can transcription proceed. One of the most critical steps in this process appears to be TFIID recruitment, which can bind sequence-specifically to the promoter at the TATA box about 30 bases upstream of the start site. This is an inefficient process at best, thus transcription factors bound to the promoter or enhancer act like magnets to attract the TFIID, which in turn recruits the other general factors and the polymerase. Similarly, transcription factors have been demonstrated to affect recruitment of almost every other general transcription factor, making this process one of the most regulated within the cell.

In the case you mention above, the 72 base repeats and the 21 base repeats are DNA sequences which are referred to as promoters/enhancers. Sp1 does bind to the 21 base repeats at the GC boxes, and appears to recruit TFIID to the promoter by contactin one of it's several subunits-specifically the 110 kDa subunit in Drosophila, but the human homolog has yet to be published.

If you need more or different info, feel free to e-mail at the address below. I hope this is useful to you.

                       -Sean
              stevens at rockvax.rockefeller.edu