¶ … Histones Face the Facts
According to the article entitled, "Histones Face the Facts," the "beads on a string" nature of eukaryotic chromatin poses a difficult quandary for scientists, in the case of RNA polymerase II complex, or RNAP II. How can this complex efficiently transcribe a coding region that is tightly wrapped in nucleosomes and why does it do so? Three recently published papers by scientists provide several key insights into the mechanisms and factors that enable transcription to take place through this chromatin, explaining why it does not have to become nucleosome free for transcription to occur.
The papers, respectively published Kaplan et al. And Belotserkovskaya et al. And Saunders et al. provide detail how the histone chaperone activity of the elongation factors Spt6 and FACT enables the dissociation of histone proteins in front of RNAPII and their immediate reassociation with the DNA in its wake. Results from the work of Kaplan, Laprade and Winston demonstrate that an spt 6 mutation impairs the integrity of chromatin in active genes. Kaplan et al.'s data strongly suggest that chromatin reassembly after passage of the polymerase is defective in cells lacking Spt6 activity, thus explaining why RNA polymerase replicates in the fashion that it does.
Secondly, Belotserkovskaya, Reinberg, and colleagues report that FACT is capable of promoting transcription dependent nucleosomes alterations, and that this factor also facilitates assembly of histone proteins into nucleosomes in vitro even in the absence of RNAPII. So these results are consistent with the idea that FACT enables chromatin structure to be disrupted and then reestablished during transcription. This model is lastly supported by work from Saunders, Lis, and co-workers showing that FACT is associated with actively transcribed RNAPII genes on Drosophila polytene chromosomes, and that its kinetics of association and site of action are consistent with its involvement in transcript elongation through chromatin. RNAPII has the intrinsic ability to disassemble nucleosomes during transcription.
In combination, all of these studies suggest that the apparently impossible technique of RNAPII is enabled through RNAP II's ability to disassemble nucleosomes and the use of FACT in transcript elongation. Interestingly enough RNAP III is not capable of this process, pointing to new, possible areas of research in the future.
