STRUCTURE AND EXPRESSION OF THE CHICKEN RETINOBLASTOMA GENE

The molecular cloning and initial characterization of the complementar y DNA encoding the chicken homologue of the human retinoblastoma susce ptibility gene product Rb are described. Chicken Rb (chRb) was found t o be highly homologous to human, mouse, and Xenopus Rb in primary amin o acid sequence, underlining its conserved function in cell cycle cont rol in higher eukaryotes. The highest level of homology was found with in the Rb ''pocket domains'' and at the Rb COOH terminus, whereas the extreme NH2 terminus of chRb is divergent. We also show that transcrip tion of chRb initiates at multiple start sites within a DNA segment wh ich represents a CpG island. ChRb is transcribed as a 4.7-kilobase (kb ) mRNA species in a variety of normal chicken tissues and oncogene-tra nsformed hematopoietic cells. However, in v-rel-transformed cell lines , a variant 3.1-kb Rb-specific mRNA was detected in addition to the 4. 7-kb transcript. ChRb is expressed as a 104 kilodalton protein and is therefore' slightly smaller than the mammalian Rb proteins.