REPLICATION OF CLONED XENOPUS LAEVIS DNA FRAGMENTS IN UNFERTILIZED EGGS AND THEIR TRANSCRIPTION IN OOCYTES

A segment of DNA from the African clawed frog (Xenopus laevis) was recently cloned into a bacterial plasmid and subsequently microinjected into unfertilized eggs of X. laevis by Watanabe and Taylor (1980). The recombinant plasmid was found to replicate with increased efficiency in unfertilized eggs than the vector itself. To further investigate the possible role of this segment of DNA ('Xori') in replication, I subcloned fragments of Xori into the bacterial plasmid pBR322. The recombinant plasmids obtained were microinjected into X. laevis eggs and their replication in vivo was studied. Comparison of the efficiencies of replication of the different recombinants and the vector confirmed that specific DNA sequences are required for the initiation of DNA replication in eukaryotes. Furthermore, Xori was found to have a sequence structure similar to that of the Alu family of mammalian repetitive sequences, some of which have been shown to be transcribed in vitro. The plasmids containing fragments of Xori were then microinjected into oocyte nuclei of X. laevis to determine if they could be transcribed. Preliminary results indicated that the recombinant plasmids were more efficiently transcribed than the vector, thus suggesting the possibility of the Alu type sequences functioning as eukaryotic origins of DNA replication.