Transcription – coupled repair

Abstract

Popravak DNA udružen s transkripcijom jedan je od putova NER, a javlja se kad RNAP zastane na oštećenju u nekodirajućem lancu. Nekodirajući lanac popravlja se puno brže nego kodirajući lanac istog gena ili neeksprimirana regija genoma. Iako je TCR prvi put uočen kod eukariota taj fenomen najbolje je okarakteriziran kod E. coli. Glavnu ulogu u TCR kod bakterija ima protein Mfd kao ATP-ovisna DNA translokaza, a poznat je i kao faktor povezan s transkripcijskim popravkom (TRCF). Model djelovanja Mfd pretpostavlja da Mfd prepoznaje zaustavljenu RNAP te da reverzno translociranu RNAP pomoću energije ATP translocira naprijed na njenu prvotnu poziciju. Zatim dolazi do otpuštanja RNAP i dovođenja enzima NER. Postoji i alternativni put Mfd-ovisnom TCR, a to je TCR u kojem otpuštanje RNAP i popravak DNA povezuje elongacijski faktor NusA. Puno se toga ulaže u poznavanje mnogo kompleksnijeg mehanizma TCR kod eukariota zbog činjenice da mutacije u genima povezanima s gubitkom TCR dovode do teških bolesti kod ljudi. Mehanizam TCR kod ljudi je uglavnom nepoznat usprkos tome što su poznati neki od glavnih sudionika tog procesa poput proteina CSA, CSB, TFIIH i XPG. Važno je ustvrditi mehanizam TCR ne samo zbog problema koje zaustavljena RNAP stvara u stanici i bolesti koje uzrokuje nefunkcionalni TCR već i zbog toga što je TRCF uključen u povezivanje centralnih staničnih procesa i što bi daljnja saznanja o načinu na koji TRCF pomoću svojih domena utječe na RNAP i NER pomoglo u otkrivanju točnog mehanizma ovog procesa.Transcription-coupled DNA repair (TCR) is one of the subpathways of nucleotide excision repair (NER) that occurs when RNA polymerase is stalled at sites of DNA damage in the template strand. Template strand is repaired more quickly that similar lesions in coding strand or unexpressed parts of genome. Although the TCR was first observed in eukaryotes TCR phenomenon is best characterized in E. coli. The main role of TCR in bacteria has Mfd (ATP-dependent DNA translocase), also known as a transcription repair coupling factor (TRCF). Model for TCR postulate that Mfd recognizes and binds specifically to stalled and reverse translocated RNAP and using energy derived from ATP hydrolysis translocates forward RNAP to the transcription block. That results in RNAP release, transcript termination and recruitment of the NER machinery to the site of lesion. Recent studies show existence of alternative Mfd-independent TCR pathway mediated by elongation factor NusA. Alot is invested in understanding of more complex mechanism of TCR in eukaryotes due to the fact that mutations in genes associated with loss of TCR lead to severe disease in humans. The mechanism of TCR in humans is largely unknown despite having known some of the major participants in this process, such as CSA, CSB, TFIIH and XPG proteins. It is important to establish the mechanism of TCR not only because of the problems that stalled RNAP causes in the cell and diseases caused by dysfunctional TCR but also because the TRCF is involved in linking central cellular processes and knowing how TRCF uses its multiple domains to manipulate RNAP and the NER would help shed the light on the functional mechanism of TCR