Mitotic transcription repression in vivo in the absence of nucleosomal chromatin condensation

All nuclear RNA synthesis is repressed during the mitotic phase of the cell cycle. In addition, RNA polymerase II (RNAP II), nascent RNA and many tran scription factors disengage from DNA during mitosis. It has been proposed t hat mitotic transcription repression and disengagement of factors are due t o either mitotic chromatin condensation or biochemical modifications to the transcription machinery. In this study, we investigate the requirement for chromatin condensation in establishing mitotic transcription repression an d factor loss, by analyzing transcription and RNAP II localization in mitot ic cells infected with herpes simplex virus type 1. We find that virus-infe cted cells enter mitosis and that mitotic viral DNA is maintained in a nucl eosome-free and noncondensed state. Our data show that RNAP II transcriptio n is repressed on cellular genes that are condensed into mitotic chromosome s and on viral genes that remain nucleosome free and noncondensed. Although RNAP II may interact indirectly with viral DNA during mitosis, it remains transcriptionally unengaged. This study demonstrates that mitotic repressio n of transcription and loss of transcription factors from mitotic DNA can o ccur independently of nucleosomal chromatin condensation.