UCSF

In eutherian mammals, dosage compensation between XX females and XY males is achieved by the transcriptional silencing of one X-chromosome in each female cell. The choice of which chromosome will be silenced is random in that both X chromosomes have an equal probability of being silenced. Despite nearly fifty years of research, the mechanisms that enable a cell to designate precisely one active and one inactive X chromosome remain elusive. What follows is an investigation into how X chromosomal fate is assigned. I present evidence that the two X chromosomes in a female cell adopt distinct states and that these states correlate with fate. I also explore the role of Xist's A-repeat in this process. The A-repeat is a highly conserved Xist element that is necessary for Xist-dependent silencing. The work within this thesis shows that the A-repeat is also required for random choice. I demonstrate that the A-repeat is important for post-transcriptional processing of Xist RNA and that the A-repeat binds the essential splicing factor ASF/SF2. In combination, these findings provide the foundation for a model in which regulation of Xist RNA splicing in ES cells is part of the stochastic process that determines which X will be inactivated in wild-type cells.