Chromatin remodeling by RNA polymerases.

Chromatin packages DNA tightly into the eukaryotic nucleus and maintains its proper functioning. Recent studies suggest the existence of two distinct mechanisms of progression of RNA polymerases through chromatin. The first is characteristic of eukaryotic RNA polymerase III, bacteriophage RNA polymerases, and probably ATP-dependent chromatin remodeling complexes. In this mechanism, nucleosomes are translocated without release of the octamer into solution. By contrast, transcription by RNA polymerase II (Pol II) involves displacement of one H2A-H2B dimer. Nucleosomes can present a barrier for transcribing Pol II that can be regulated in vivo. Analysis of the mechanisms of transcription through chromatin should provide important information about mechanisms of chromatin remodeling and gene regulation at the level of transcript elongation.