A Ubiquitin Carrier Protein cDNA from Developing Alfalfa Embryos

Ubiquitin, a low mo1 wt protein, is present in eukaryotic tissues, either in a free state or covalently linked to a variety of cytoplasmic, nuclear, or membrane proteins (Stefanet al., 1987). It is used in the selective degradation of abnormal or short-lived proteins (Hershko, 1988). In addition to its association with proteolytic activity, it is involved in other functions such as DNA repair, cell surface recognition, and the regulation of chromatin structure through ubiquinated histone proteins H2A and H2B (Goldknopf and Busch, 1977). Conjugation of ubiquitin with other proteins involves a sequential reaction of requiring ATP and ubiquitin-activating enzyme; the conjugate is then transferred to ubiquitin-carrier proteins. These activated proteins donate ubiquitin to the specific substrate to be modified (Chau et al., 1989). We have cloned and sequenced a ubiquitin carrier protein from developing alfalfa (Medicago sativa) zygotic embryos. Initial screening of a pUC13 library yielded a clone that appeared to have homology with the storage-protein-specific clone for both sulfur-rich 2s protein from Brazil nut (Gander et al., 1991) and leg B from Vicia faba (Baumlein et al., 1986). The 5’ untranslated part of the isolated clone had almost an 85% homology with the Brazil nut 2s protein, and the first 200 nt in reverse orientation have a close homology with leg B from V. faba. Sequence comparisons using the BLAST (Basic Local Alignment Search Tools) program (Altschul et al., 1990) from National Center for Biotechnology Information, (U.S.) National Institutes of Health, indicated that the isolated cione is a ubiquitin carrier protein. See Table I for details.