Feb. 2, 2017
A study carried out by Frédéric Coin’s team has challenged one of the mechanisms of the initiation of gene transcription in eukaryotes. These results are published on February 2nd in the journal Molecular Cell.
Transcription and its mechanisms
Transcription allows a gene made of DNA to be copied into RNA. The enzyme involved in this mechanism is called RNA polymerase. In eukaryotes, three types of RNA polymerases are distinguished. While RNA polymerases I and III respectively carry out transcription of genes encoding ribosomal RNA and small non-coding RNAs, RNA polymerase II transcribes the messenger RNAs encoding the proteins. Since DNA is a double-stranded molecule, one of the first steps of transcription consists in the opening of this double helix at promoter sequences in order to allow the polymerase to have access to the nucleic acid bases. Based on current knowledge, RNA polymerase II is different from the other two eukaryotic RNA polymerases but also from prokaryotic RNA polymerases because it needs a co-factor called XPB, forming part of the transcription complex TFIIH, to perform this opening step. The current model suggests that XPB is a "helicase" using ATP energy to unwind double stranded DNA into two single strands at the gene promoter. The proposed mechanism is reinforced by the following observation: triptolide, a small molecule that targets the ATPase activity of XPB, inhibits transcription.
RNA polymerase II: a mechanism not so different
In order to refine the role of XPB during transcription, researchers induced its direct degradation through a drug, the spironolactone. To their surprise, the transcription was not affected and even became insensitive to the inhibition by triptolide in these conditions. The transcription of the messenger RNAs would therefore be blocked when the ATPase activity of XPB is inhibited, but could work without XPB… These observations show that the mechanism of promoter opening involving XPB as a helicase is not correct, which led the researchers to propose an alternative model of XPB operating in transcription. By using mutants in the helicase domains of XPB in vitro, the team of Frédéric Coin has thus demonstrated that XPB was intrinsically inhibitor of the opening of the promoters in the absence of ATP. It is only when using the ATP that it moves along the promoter and releases it to allow its opening. These data show that the mechanism of opening of the promoters by the RNA polymerase II does not require a helicase activity, and that it is finally carried out in the same way as with the other RNA polymerases. XPB inhibitory control would be just another way to regulate gene expression.
Since XPB also has a key role in the mechanism of DNA repair, its removal through spironolactone is proposed to improve the effectiveness of chemotherapies. This new knowledge on the precise role of XPB in the transcription process suggests the development of even more targeted therapies, targeting only one of its mechanisms of action.