Structure and expression of the Xenopus retinoblastoma gene

We have cloned a Xenopus homology (XRb1) of the human retinoblastoma susceptibility gene. DNA sequence analysis shows that the XRb1 gene product is highly conserved in many regions. The leucine repeat motif and many of the potential cdc2 phosphorylation sites, as well as potential sites for other kinases, are retained. The region of the protein homologous to the SV40 T antigen binding site and the basic region directly C-terminal to the E1A binding site are all conserved. XRb1 gene expression at the RNA level was studied by Northern blot analysis. Transcripts of 4.2 and 10-kb are present as maternal RNA stores in the oocyte. While the 4.2-kb product is stable until at least the mid-blastula stage, the 10-kb transcript is selectively degraded. Between stages 11 and 13 the 10-kb transcript reappears and also a minor product of approximately 11 kb becomes apparent. Both the 4.2- and the 10-kb transcripts remain present until later stages of development and are also present in all adult tissues examined, although at differing levels. Antibodies raised against human p105Rb which recognize the protein product of the XRb1 gene, pXRb1, detect the Xenopus 99-kDa protein prior to the mid-blastula stage, but at lower levels than at later stages in development.