Dynamics of RNA polymerase II localization during the cell cycle

Mitosis is characterized by condensation of chromatin, cessation of RNA tra nscription, and redistribution of nuclear proteins. We investigated the dis tribution of the hypo- and hyperphosphorylated forms of RNA polymerase II i n mitotic cells from different cell lines by immunofluorescence. In interph ase cells, the hyperphosphorylated RNA polymerase II (Pol IIO) is present i n speckles and diffusely throughout the nucleoplasm. In prophase, when spec kles disappear, Pol IIO concentrates at the surface of chromosomes and, in addition, localizes in small spots throughout the cytoplasm. The associatio n of Pol IIO with the surface of chromosomes is visible until the chromosom es start to decondense during late anaphase/early telophase. In telophase c ells, Pol IIO is absent in newly formed nuclei but present in the cytoplasm , while Pol IIO disappears nearly completely in late telophase cells. In ea rly G1 cells, when cell nuclei increase in size, Pol IIO becomes present in the nucleus, first in small spots and later diffusely and in speckles. The hypophosphorylated form of RNA polymerase II (Pol IIA) is nearly absent in mitotic cells suggesting that Pol IIA is hyperphosphorylated at the onset of mitosis. Because Pol IIO, unlike Pol IIA, cannot assemble in transcripti on preinitiation complexes, the conversion of Pol IIA to Pol IIO and the li ning of chromosomes with Pol IIO might be underlying a mechanism by which m itotic cells repress their transcriptional activity.