Question

Why is TBP considered to be a universal transcription factor?

Short answer

A TBP mutant that is deficient for TATA box-binding can nonetheless facilitate in vitro transcriptional initiation by both RNAP I and RNAP III with certain class II TATA-less promoters. Therefore, TBP is the only known universal transcription factor.

Step-by-step solution

Introduction

To initiate transcription at their respective promoters, RNAP I and RNAP III require different sets of GTFs from each other and from RNAP II. This is not surprising, given the three types of promoters' highly diverse organizational structures. TATA boxes are absent from nearly all promotersrecognized by RNAP I (class I promoters) and nearly all promoters recognized by RNAP III (class III promoters).

TBP as a universal transcription factor

When it was discovered that TBP is required for both RNAP I and RNAP III initiation, it came as a shock. TBP participates by forming the GTFs SLI (with class I promoters) and TFIIIB (with class III promoters) by combining with various sets of TAFs. A TBP mutant that is faulty for TATA box-binding, like certain class II TATA-less promoters, can still support in vitro transcriptional initiation by both RNAP I and RNAP III. TBP, the only known universal transcription factor, is clearly a protein with a wide range of applications.