Retrotransposons and regulatory suites.

Cellular differentiation and multicellular development require the programmed expression of coregulated suites of genetic loci dispersed throughout the genome. How do functionally diverse loci come to share common regulatory motifs? A new paper finds that retrotransposons (RTEs) may play a role in providing common regulation to a group of functions expressed during the development of oocytes and preimplantation embryos. Examining cDNA libraries, Peaston et al. find that 13% of all processed transcripts in full-grown mouse oocytes contain RTE sequences, mostly from the MT family of retroviral-like elements. Smaller but still significant percentages of RTE sequences are found in cDNA libraries from 2-cell embryos and blastocysts. A quarter of these RTE sequences are at the 5' ends of chimeric transcripts that also contain exons from endogenous mouse loci. These chimeric transcripts display restricted expression in oocytes and preimplantation embryos and presumably originate from developmentally regulated LTR promoters. Some, but not all, chimeric transcripts encode novel protein products.