The yeast ubiquitin genes: a family of natural gene fusions.

Ubiquitin is a 76-residue protein highly conserved among eukaryotes. Conjugation of ubiquitin to intracellular proteins mediates their selective degradation in vivo. We describe a family of four ubiquitin-coding loci in the yeast Saccharomyces cerevisiae. UB11, UB12 and UB13 encode hybrid proteins in which ubiquitin is fused to unrelated ('tail') amino acid sequences. The ubiquitin coding elements of UB11 and UB12 are interrupted at identical positions by non-homologous introns. UB11 and UB12 encode identical 52-residue tails, whereas UB13 encodes a different 76-residue tail. The tail amino acid sequences are highly conserved between yeast and mammals. Each tail contains a putative metal-binding, nucleic acid-binding domain of the form Cys-X2-4-Cys-X2-15-Cys-X2-4-Cys, suggesting that these proteins may function by binding to DNA. The fourth gene, UB14, encodes a polyubiquitin precursor protein containing five ubiquitin repeats in a head-to-tail, spacerless arrangement. All four ubiquitin genes are expressed in exponentially growing cells, while in stationary-phase cells the expression of UB11 and UB12 is repressed. The UB14 gene, which is strongly inducible by starvation, high temperatures and other stresses, contains in its upstream region strong homologies to the consensus 'heat shock box' nucleotide sequence. Elsewhere we show that the essential function of the UB14 gene is to provide ubiquitin to cells under stress.