It’s summertime—a time when you just want to take things slow. Researchers now report that 5-formylcytosines (5fCs) and 5-carboxylcytosines (5caCs) like to slow things down, no matter what the season. They say that these mods put the brakes on RNA Pol II transcription and make it less specific.
Sure, researchers have known about 5fC and 5caC for a little while now, though 5-methylcytosine (5mC) usually steals the cytosine modification spotlight. These two mods have been proposed to have roles in the cytosine demethylation pathway. But until now, no one knew what role they might have in transcription.
The team from UCSD and U of Chicago compared Pol II elongation efficiencies on DNA templates that had a site-specific unmodified cytosine (C), 5mC, 5-hydroxymethylcytosine (5hmC), 5fC, or 5caC. It turned out that templates with either 5fC or 5caC had much lower GTP incorporation efficiencies than the others. Another clue that things slow down at 5fCs and 5caCs was the presence of a lot of backtracked Pol II complexes. They say that this result suggests that Pol II shifts from an active population to a paused one that can stall and backtrack on 5fC- and 5caC-containing DNA. Pol II specificity for GTP also was significantly reduced on templates with these mods. And in another set of experiments, 5fC greatly reduced the fidelity of nucleotide incorporation.
All of this may mean that these mods could be important players in transcriptional regulation, fine-tuning things. 5fC- and 5caC-mediated pausing also could serve as a signal to recruit other factors to the DNA, say the researchers.
Slow down and browse through the paper at Nature Structural & Molecular Biology, July 2012.