The yeast ubiquitin gene: head-to-tail repeats encoding a polyubiquitin precursor protein

Abstract

Ubiquitin¹, a 76-residue protein, occurs in cells either free or covalently joined to a variety of protein species^2,3, from chromosomal histones^4–6 to cytoplasmic proteins. Conjugation of ubiquitin to proteolytic substrates is essential for the selective degradation of intracellular proteins in higher eukaryotes^7,8. We show here that a protein homologous to human ubiquitin exists in the yeast Saccharomyces cerevisiae, and that yeast extracts conjugate human ubiquitin to a variety of endogenous proteins in an ATP-dependent reaction. We have isolated the S. cerevisiae ubiquitin gene and found it to contain six consecutive ubiquitin-coding repeats in a head-to-tail arrangement. This apparently unique gene organization suggests that yeast ubiquitin is generated by processing of a precursor protein in which several exact repeats of the ubiquitin amino acid sequence are joined directly via Gly–Met peptide bonds between the last and first residues of mature ubiquitin, respectively. Ubiquitin-coding yeast DNA repeats are restricted to a single genomic locus; although the sequenced repeats differ in up to 27 of 228 bases per repeat, they encode identical amino acid sequences. As this predicted amino acid sequence differs in only 3 of 76 residues from that of ubiquitin in higher eukaryotes, ubiquitin is apparently the most conserved of known proteins.