Transcription of a class II gene starts at a guanosine 25 bp downstream of the last base of the

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Transcription of a class II gene starts at a guanosine 25 bp...

Transcription of a class II gene starts at a guanosine $25 \mathrm{bp}$ downstream of the last base of the TATA box. You delete 20 bp of DNA between this guanosine and the TATA box and transfect cells with this mutated DNA. Will transcription still start at the same guanosine? If not, where? How would you locate the transcription start site?

Key Concepts

Promoter Architecture and the TATA Box

The TATA box is a core promoter element found in many class II genes that helps in correctly positioning the transcriptional machinery. It serves as a binding site for the TATA-binding protein (TBP), a component of the transcription factor IID complex, which is critical for the recruitment of RNA polymerase II and the assembly of the transcription initiation complex.

Spacing Between the TATA Box and the Transcription Start Site

The distance between the TATA box and the transcription start site (TSS) is crucial for the proper assembly of the transcription initiation complex. Typically, transcription initiates at a fixed distance downstream (often about 25-30 bp) from the TATA box. Alterations in this spacing, such as a deletion within this region, can shift the position where the transcription complex forms and initiates RNA synthesis.

Effect of Deletions on Transcription Initiation

Deletions in the region between the TATA box and the TSS may disrupt the normal spacing required for the correct positioning of RNA polymerase II. When such a deletion occurs, the transcription start site may shift to a site that maintains the optimal spacing relative to the TATA box. This shift is due to the transcriptional machinery's need to correctly position itself, and thus it may choose a new start site that is approximately the same distance downstream of the TATA box as in the wild-type configuration.

Techniques for Mapping Transcription Start Sites

To experimentally determine the transcription start site, methods such as primer extension, 5’ rapid amplification of cDNA ends (5' RACE), and RNAse protection assays are commonly used. These techniques allow researchers to precisely identify the 5’ end of the mRNA transcript by mapping the location where transcription begins, thereby providing insight into how mutations or deletions in promoter regions affect transcription initiation.

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