Characterization of repetitive DNA transcripts isolated from a Xenopus laevis gastrula-stage cDNA clone bank

By screening a cDNA library prepared from polyadenylated nuclear RNA of Xenopus laevis gastrula-stage embryos with total genomic DNA we have identified nine clones belonging to seven different families of repeated DNA. Two of these families exhibit partial sequence homologies and thus probably represent subfamilies of a common origin. The individual families comprise between 0.024% and 1.5% and, in total, about 2% of the X. laevis genome. Six of the clones elements represent dispersed repetitive DNA families; the seventh is mainly organized in a tandem fashion. Transcripts containing one type of repeat do not appear before the blastula stage and then accumulate in the course of development; however, the others can be detected within the unfertilized egg and their amount remains fairly constant throughout embryogenesis.