Rsx is a metatherian RNA with Xist-like properties in X-chromosome inactivation.

In female (XX) mammals, one of the two X chromosomes is inactivated to ensure an equal dose of X-linked genes with males (XY). X-chromosome inactivation in eutherian mammals is mediated by the non-coding RNA Xist. Xist is not found in metatherians (marsupials), and how X-chromosome inactivation is initiated in these mammals has been the subject of speculation for decades. Using the marsupial Monodelphis domestica, here we identify Rsx (RNA-on-the-silent X), an RNA that has properties consistent with a role in X-chromosome inactivation. Rsx is a large, repeat-rich RNA that is expressed only in females and is transcribed from, and coats, the inactive X chromosome. In female germ cells, in which both X chromosomes are active, Rsx is silenced, linking Rsx expression to X-chromosome inactivation and reactivation. Integration of an Rsx transgene on an autosome in mouse embryonic stem cells leads to gene silencing in cis. Our findings permit comparative studies of X-chromosome inactivation in mammals and pose questions about the mechanisms by which X-chromosome inactivation is achieved in eutherians.

Widespread transcription in an amphibian oocyte relates to its reprogramming activity on transplanted somatic nuclei.

Amphibian oocytes have the special ability to directly induce the transcription of pluripotency and other genes in transplanted somatic nuclei. To this extent, oocytes induce a stem cell-like pattern of transcription in somatic cell nuclei. We ask whether the induced transcription in transplanted nuclei reflects the normal transcriptional activity of oocyte genes. We describe here the transcript content of a wide range of genes in Xenopus tropicalis oocytes. Using accurate quantitation, we find that each mature oocyte has accumulated several hundred transcripts of cell-type specific genes. This value is several orders of magnitude greater than the "leakage" level found in most somatic cells and about the same level found in somatic cells where these genes are fully expressed. Illumina sequencing confirms the high transcript content of a mature Xenopus oocyte. Most of the transcripts from these highly expressed genes in oocytes are correctly and efficiently spliced. Our results contribute a more quantitative view of certain amphibian oocyte transcripts than previously available. Our results also show that transplanted somatic nuclei conform, with respect to the genes analyzed, to the transcriptional characteristics of the recipient oocytes.