Two ubiquitin-long-tail fusion genes arranged as closely spaced direct repeats in barley.

Ubiquitin (Ubi) genes encode two types of fusion proteins: polyUbi with a varying number of direct repeats of Ubi, and Ubi-tail fusions with long or short basic C-terminal extensions. A barley (Hordeum vulgare) genomic clone has been isolated with two very similar, intronless genes encoding monoUbi-long-tail fusion peptides. The genes are arranged as direct repeats separated by 3 kb of DNA and account for two of the probable three long-tail genes in the haploid barley genome. Both genes are active and give rise to messengers about 800 nt long. The sequence of the encoded Ubi moieties is identical to the sequence of Ubi repeats of polyUbi precursors from barley and other plants. The basic tails of the peptides are 79 aa long and 71-72% homologous to corresponding sequences from yeast and man. Recently, it was found that the long and short tails are ribosomal proteins in yeast [Finley et al., Nature 338 (1989) 394-401] and the evolutionary conservation of the structure of the Ubi-tail fusion genes suggests that they serve the same function in plants. The similarity between yeast and barley Ubi-long-tail fusion genes may extend to the regulatory regions, since upstream activating sites characteristic of ribosomal protein-encoding genes in yeast (UASrpg) were found in the barley genes.