To cross or not to cross the nucleosome, that is the elongation question

The natural template for transcription is chromatin. In vitro and in vivo experiments demonstrate that positioned nucleosomes are obstacles for RNA polymerase II (RNAPII) elongation, raising the question of how RNAPII crosses a nucleosome. In fact, transcription elongation is accompanied by chromatin remodeling in the body of the genes. Numerous results evidence that chromatin remodelers such as histone chaperones and histone acetyl transferases contribute to transcription elongation. Recent data indicate that the SWI/SNF complex, an ATP-dependent chromatin remodeling machine, also helps RNAPII to overcome a nucleosomal barrier during elongation. Finally, the idea that remodeling of positioned nucleosomes in the coding regions would alter RNAPII elongation rate and, therefore, would regulate gene expression at different levels is discussed.