Meta-Biol

• Pathways

E2F6, similar to other E2F proteins, possesses the DNA binding domain, the dimerization domain and the marked box. E2F6, however, does not have a pocket protein binding domain and thus does not interact with the retinoblastoma family members RB1, RBL1 (p107) and RBL2 (p130) (Gaubatz et al. 1998, Trimarchi et al. 1998, Cartwright et al. 1998). E2F6 lacks the transactivation domain and acts as a transcriptional repressor (Gaubatz et al. 1998, Trimarchi et al. 1998, Cartwright et al. 1998). E2F6 forms a heterodimer with TFDP1 (DP-1) (Trimarchi et al. 1998, Ogawa et al. 2002, Cartwright et al. 1998) or TFDP2 (DP-2) (Gaubatz et al. 1998, Trimarchi et al. 1998, Cartwright et al. 1998).

E2f6 knockout mice are viable and embryonic fibroblasts derived from these mice proliferate normally. Although E2f6 knockout mice appear healthy, they are affected by homeotic transformations of the axial skeleton, involving vertebrae and ribs. Similar skeletal defects have been reported in mice harboring mutations in polycomb genes, suggesting that E2F6 may function in recruitment of polycomb repressor complex(es) to target promoters (Storre et al. 2002).

E2F6 mediates repression of E2F responsive genes. While E2F6 was suggested to maintain G0 state in quiescent cells (Gaubatz et al. 1998, Ogawa et al. 2002), this finding has been challenged (Giangrande et al. 2004, Bertoli et al. 2013, Bertoli et al. 2016). Instead, E2F6-mediated gene repression in proliferating (non-quiescent) cells is thought to repress E2F targets involved in G1/S transition during S phase of the cell cycle. E2F6 does not affect E2F targets involved in G2/M transition (Oberley et al. 2003, Giangrande et al. 2004, Attwooll et al. 2005, Trojer et al. 2011, Bertoli et al. 2013). In the context of the E2F6.com-1 complex, E2F6 was shown to bind to promoters of E2F1, MYC, CDC25A and TK1 genes (Ogawa et al. 2002). E2F6 also binds the promoters of CDC6, RRM1 (RR1), PCNA and TYMS (TS) genes (Giangrande et al. 2004), as well as the promoter of the DHFR gene (Gaubatz et al. 1998). While transcriptional repression by the E2F6.com 1 complex may be associated with histone methyltransferase activity (Ogawa et al. 2002), E2F6 can also repress transcription independently of H3K9 methylation (Oberley et al. 2003).

During S phase, E2F6 is involved in the DNA replication stress checkpoint (Bertoli et al. 2013, Bertoli et al. 2016). Under replication stress, CHEK1-mediated phosphorylation prevents association of E2F6 with its target promoters, allowing transcription of E2F target genes whose expression is needed for resolution of stalled replication forks and restart of DNA synthesis. Inability to induce transcription of E2F target genes (due to CHEK1 inhibition or E2F6 overexpression) leads to replication stress induced DNA damage (Bertoli et al. 2013, Bertoli et al. 2016). E2F6 represses transcription of a number of E2F targets involved in DNA synthesis and repair, such as RRM2, RAD51, BRCA1, and RBBP8 (Oberley et al. 2003, Bertoli et al. 2013).

RNA polymerase II (Pol II) is the central enzyme that catalyses DNA- directed mRNA synthesis during the transcription of protein-coding genes. Pol II consists of a 10-subunit catalytic core, which alone is capable of elongating the RNA transcript, and a complex of two subunits, Rpb4/7, that is required for transcription initiation.
The transcription cycle is divided in three major phases: initiation, elongation, and termination. Transcription initiation include promoter DNA binding, DNA melting, and initial synthesis of short RNA transcripts. The transition from initiation to elongation, is referred to as promoter escape and leads to a stable elongation complex that is characterized by an open DNA region or transcription bubble. The bubble contains the DNA-RNA hybrid, a heteroduplex of eight to nine base pairs. The growing 3-end of the RNA is engaged with the polymerase complex active site. Ultimately transcription terminates and Pol II dissocitates from the template.

Gene expression encompasses transcription and translation and the regulation of these processes. RNA Polymerase I Transcription produces the large preribosomal RNA transcript (45S pre-rRNA) that is processed to yield 18S rRNA, 28S rRNA, and 5.8S rRNA, accounting for about half the RNA in a cell. RNA Polymerase II transcription produces messenger RNAs (mRNA) as well as a subset of non-coding RNAs including many small nucleolar RNAs (snRNA) and microRNAs (miRNA). RNA Polymerase III Transcription produces transfer RNAs (tRNA), 5S RNA, 7SL RNA, and U6 snRNA. Transcription from mitochondrial promoters is performed by the mitochondrial RNA polymerase, POLRMT, to yield long transcripts from each DNA strand that are processed to yield 12S rRNA, 16S rRNA, tRNAs, and a few RNAs encoding components of the electron transport chain. Regulation of gene expression can be divided into epigenetic regulation, transcriptional regulation, and post-transcription regulation (comprising translational efficiency and RNA stability). Epigenetic regulation of gene expression is the result of heritable chemical modifications to DNA and DNA-binding proteins such as histones. Epigenetic changes result in altered chromatin complexes that influence transcription. Gene Silencing by RNA mostly occurs post-transcriptionally but can also affect transcription. Small RNAs originating from the genome (miRNAs) or from exogenous RNA (siRNAs) are processed and transferred to the RNA-induced silencing complex (RISC), which interacts with complementary RNA to cause cleavage, translational inhibition, or transcriptional inhibition.