Saving a host cell from itself: A fundamental mammalian defense mechanism against pathogens and damaged cellular DNA is to recognize DNA fragments in the cytosol and trigger an inflammatory response. The cyclic guanosine monophosphate–adenosine monophosphate synthase (cGAS) that recognizes cytosolic DNA is also found in the nucleus, but here its activity is suppressed by tethering to chromatin. Two papers now report cryo–electron microscopy structures of cGAS bound to the nucleosome core particle (NCP). Kujirai et al. observed a structure with two cGAS molecules bridging two NCPs, whereas Boyer et al. observed cGAS bound to a single nucleosome. Together, these structures show how cGAS is prevented from autoreactivity toward host DNA. Science , this issue p. 455 , p. 450

Nucleosomal DNA transcription: In eukaryotes, the basic chromatin unit nucleosome stalls RNA polymerase II (RNAPII) when it transcribes genetic information on DNA. Using cryo–electron microscopy, Kujirai et al. explored seven structures of the RNAPII-nucleosome complex, in which RNAPII pauses at four locations on the nucleosome. These serial snapshots of the RNAPII progression reveal the molecular mechanism of how RNAPII peels the nucleosomal DNA off the histone stepwise. Science , this issue p. 595

Getting over nucleosomal barriers: In eukaryotic cells, RNA polymerase II (RNAPII) transcribes DNA within nucleosome-coated chromatin. The nucleosomes can provide major roadblocks for transcription. Cells solve this problem by using transcription elongation factors. Ehara et al. solved the cryo–electron microscopy structures of the nucleosome-transcribing RNAPII with elongation factors Elf1 and Spt4/5. Elf1 and Spt4/5 cooperatively suppress RNAPII pausing at multiple super helical locations [SHL(−6), SHL(−5), and SHL(−2)] and facilitate RNAPII progression through SHL(−1) by adjusting the nucleosome position to favor forward progression. Science , this issue p. 744

During gene transcription, RNA polymerase II (RNAPII) traverses nucleosomes in chromatin, but the mechanism has remained elusive. Using cryo–electron microscopy, we obtained structures of the RNAPII elongation complex (EC) passing through a nucleosome in the presence of the transcription elongation factors Spt6, Spn1, Elf1, Spt4/5, and Paf1C and the histone chaperone FACT (facilitates chromatin transcription). The structures show snapshots of EC progression on DNA mediating downstream nucleosome disassembly, followed by its reassembly upstream of the EC, which is facilitated by FACT. FACT dynamically adapts to successively occurring subnucleosome intermediates, forming an interface with the EC. Spt6, Spt4/5, and Paf1C form a “cradle” at the EC DNA-exit site and support the upstream nucleosome reassembly. These structures explain the mechanism by which the EC traverses nucleosomes while maintaining the chromatin structure and epigenetic information.

Eukaryotic genomic DNA is wrapped around the histone octamer and forms the nucleosome, which is the basic unit of chromatin. Nevertheless, eukaryotic RNA polymerases transcribe the DNA that is tightly bound to the histone core in the nucleosome. For transcription to proceed on the nucleosome, the RNA polymerases must overcome the tight contacts between histones and DNA during transcription elongation. Here, we review the structural and biochemical studies of the transcription mechanism on the nucleosome, and focus on recent information to understand how RNA polymerases transcribe the genomic DNA in chromatin.

The metazoan transcription elongation complex (EC) of RNA polymerase II (RNAPII) generally stalls between the transcription start site and the first (+1) nucleosome. This promoter-proximal pausing involves negative elongation factor (NELF), 5,6-dichloro-1-β- d -ribobenzimidazole sensitivity-inducing factor (DSIF), and transcription elongation factor IIS (TFIIS) and is critical for subsequent productive transcription elongation. However, the detailed pausing mechanism and the involvement of the +1 nucleosome remain enigmatic. Here, we report cryo–electron microscopy structures of ECs stalled on nucleosomal DNA. In the absence of TFIIS, the EC is backtracked/arrested due to conflicts between NELF and the nucleosome. We identified two alternative binding modes of NELF, one of which reveals a critical contact with the downstream DNA through the conserved NELF-E basic helix. Upon binding with TFIIS, the EC progressed to the nucleosome to establish a paused EC with a partially unwrapped nucleosome. This paused EC strongly restricts EC progression further downstream. These structures illuminate the mechanism of RNAPII pausing/stalling at the +1 nucleosome.