Complementary DNA clones encoding ubiqultin fused to a 52 amino acid tall protein were Isolated from human placental and adrenal gland cDNA libraries. The deduced human 52 amino acid tail protein is very similar to the homologous protein from other species, including the conservation of the putative metal-binding, nucleic acid-binding domain observed in these proteins. Northern blot analysis with a tail-specific probe indicated that the previously identified UbA mRNA species most likely represents comigrating transcripts of the 52 amino acid tall (UbA and 80 amino acid tall (UbA80) ubiqultin fusion genes. The UbA52 gene was isolated from a human genomic library and consists of five exons distributed over 3400 base pairs. One intron is in the 5' non-coding region, two interrupt the single ubiquitin coding unit, and the fourth Intron is within the tail coding region. Several members of the Alu family of repetitive DNA are associated with the gene. The UbA52 promoter has several features in common with mammalian ribosomal protein genes, including its location in a CpG-rlch island, Initiation of transcription within a polypyilmidine tract, the lack of a consensus TATA motif, and the presence of Spi binding sites, observations that are consistent with the recent Identification of the ubiqultin free tail proteins as ribosomal proteins. Thus, in spite of Its unusual feature of being translationally fused to ubiquitin, the 52 amino acid tail ribosomai protein is expressed from a structurally typical ribosomal protein gene.

Author notes

+Present address: Department of Biology, Room 16-520, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.