Eukaryotic pol II is more complex than bacterial RNAP (Eσ) as it consists of 12 subunits. Surprisingly though, the two largest subunits of pol II (MW 220 and 150 kDa) are partially homologous in sequence (and structure) to the bacterial β′ and β subunits. The most striking difference between eukaryotic pol II and its prokaryotic counterpart in that pol II has a series of heptad repeats with consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser (YSPTSPS) at the carboxyl terminus of the largest pol II subunit, the so-called C-terminal domain(CTD). The length of this CTD can vary from organ ism to organism; for example, there are 26 repeated units in yeast and 52 units in mammals. The CTD is a substrate for several enzymes (kinases, phosphatases, prolyl isomerases, glycosylases); phosphorylation of the CTD was the first CTD PTM discovered. Among other proteins the kinase subunit of TFIIH can modify the CTD. Covalently modified CTD is the binding site, or platform, for a wide array of proteins, and it has been shown to interact with many transcription regulators, mRNA modifying and processing enzymes, and nuclear transport proteins. The association of these factors with the CTD of RNA polymerase II (and other components of the basal machinery) thus serves to couple transcription initiation with mRNA capping, splicing, 3′–end formation, mRNA quality control, and transport to the cytoplasm. Pol II polymerization is activated when phosphorylated on the Ser and Thr residues and displays reduced activity when the CTD is dephosphorylated. CTD phosphorylation/dephosphorylation is critical for promoter clearance, elongation, termination, and even appropriate mRNA processing. Genetically modified Pol II lacking the CTD tail is incapable of activating transcription in vitro, while cells expressing pol II lacking the CTD rapidly lose viability. These results underscore the importance of this domain to mRNA biogenesis and metabolism.

Pol II can associate with other proteins termed mediator or Med proteins to form a complex sometimes referred to as the pol II holoenzyme; this complex can form on the promoter or in solution prior to PIC formation. The Med proteins (more than 30 proteins; Med1-Med31) are essential for appropriate regulation of pol II transcription by serving myriad roles, both activating and repressing transcription. Thus, mediator, like TFIID is a transcriptional coregulator (Figure 1).

Fig1. Models for the formation of an RNA polymerase II preinitiation complex. Shown at top is a typical mRNA gene transcription unit: enhancer-promoter (TATA)-TSS (bent arrow) and ORF (open reading frame; sequences encoding the protein) within the transcribed region. PICs have been shown to form by at least two distinct mechanisms in vitro: (A) the stepwise binding of GTFs, pol II, and mediator (Med), or (B) by the binding of a single multiprotein complex composed of pol II, Med, and the six GTFs. DNA-binding transactivator proteins specifically bind enhancers and in part facilitate PIC formation (or PIC function) by binding directly to the TFIID-TAF subunits, or Med subunits of mediator (not shown, see Figure 36–10), or other components of the transcription machinery. The molecular mechanism(s) by which such protein–protein interactions stimulate transcription remain a subject of intense investigation.

اخر الاخبار

اخبار العتبة العباسية المقدسة

جامعة العميد تنظم ورشة تدريبية عن برنامج منصة التعليم العالي

قسم الشؤون الفكرية ينظم برنامجًا دينيًّا لكشافة الكفيل في النجف الأشرف

العتبة العباسية المقدسة تقدم دعوة لجامعتي الكوفة والطوسي للمشاركة في مشروعها القرآني الخاص بطلبة الجامعات

قسم الشؤون الفكرية يصدر مجموعة جديدة من الإصدارات المعرفية والثقافية

المزيد

الآخبار الصحية

كيف يمكن لحفنة من الجوز أن تؤثر على نومك وتركيزك؟

دراسة: كوب ميلك شيك واحد قد يكون "قنبلة دماغية"

هل يعد "الباراسيتامول" خيارا آمنا للنساء الحوامل؟

دراسة تكشف الرابط بين العلاقات العاطفية والرضا عن الحياة

المزيد

الآخبار التكنلوجية

كارثة صامتة.. "الهواء الملوث" يسرق نور عيون الأطفال

دراسة: رصد أول دليل على وجود "إشعاع هوكينغ"

رسالة بلغة لا تعرفها؟.. "واتساب" يترجمها لك فورا

"إنستغرام".. ميزة جديدة لكشف "الكذب في العمر"

المزيد

مواضيع ذات صلة

RNA Molecules are Extensively Processed Before They Become Functional

Formation of the Pol II Transcription Complex

Eukaryotic Promoters Are More Complex

Bacterial Promoters Are Relatively Simple

RNA Synthesis is A Cyclical Process that Involves RNA Chain Initiation, Elongation, & Termination

RNA is Synthesized From A DNA Template by RNA Polymerases

DNA & Chromosome Integrity Is Monitored Throughout the Cell Cycle

All Organisms Contain Elaborate Evolutionarily Conserved Mechanisms to Repair Damaged DNA

Micro-RNA and Long Noncoding RNA

Histone Organization

Noncoding DNA

DNA Synthesis Occurs During the S Phase of the Cell Cycle