Ubiquitin extension proteins of Arabidopsis thaliana. Structure, localization, and expression of their promoters in transgenic tobacco.

The highly conserved protein ubiquitin is synthesized in eukaryotes as two types of protein fusions from which active ubiquitin is derived by proteolytic processing. We report here the isolation and characterization of multiple genes from one type that encode ubiquitin extension proteins from the higher plant, Arabidopsis thaliana (L.). Two genes with 90% nucleotide identity in their exons encode ubiquitin and identical 52-amino acid (aa) extension proteins with 85 and 79% aa identity to 52-aa extension proteins from humans and yeast, respectively. Two other genes with 90% nucleotide identity encode ubiquitin and 81-aa extension proteins that differ by 4 amino acids from each other and are approximately 70% identical to the 76- and the 80-aa extension proteins from yeast and humans, respectively. Antibodies recognizing the 52- and 81-aa Arabidopsis extension proteins identify them as constituents of ribosomes. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the 52- and 81-aa extension proteins migrate at 6.8 and 11.5 kDa, respectively, and neither cross-reacts with anti-ubiquitin antibodies, indicating that extension proteins are cleaved from ubiquitin following translation. Ubiquitin extension protein genes encode the smallest transcript size class of ubiquitin mRNAs in Arabidopsis. The 5'-flanking regions of both UBQ1 and UBQ6, genes representative of the both extension proteins, direct the expression of readily detectable levels of the marker enzyme beta-glucuronidase in transgenic tobacco, suggesting the utility of these promoters for expression of foreign genes in higher plants.