The human ubiquitin multigene family: some genes contain multiple directly repeated ubiquitin coding sequences

Ubiquitin coding sequences were isolated from a human genomic library and two cDNA libraries. One human ubiquitin gene consists of 2055 nucleotides and codes for a polyprotein consisting of 685 amino acid residues. The polyprotein contains nine direct repeats of the ubiquitin amino acid sequence and the last ubiquitin sequence is extended with an additional valyl residue at the C-terminal end. No spacer sequences separate the ubiquitin repeats and the coding regions are not interrupted by intervening sequences. This particular gene is transcribed since cDNAs corresponding to the genomic sequence have been isolated. At least two more types of ubiquitin genes are encoded in the human genome, one coding for an ubiquitin monomer while another presumably codes for three or four direct repeats of the ubiquitin sequence. Human DNA contains many copies of the ubiquitin sequence. Ubiquitin is therefore encoded in the human genome as a multigene family.