A single protein comprised of tandem repeats of the UBIQUITIN 78-amino acid sequence. It is a product of the polyubiquitin gene which contains multiple copies of the ubiquitin coding sequence. Proteolytic processing of ubiquitin c results in the formation of individual ubiquitin molecules. This protein is distinct from polyubiquitin, which is a protein formed through isopeptide linkage of multiple ubiquitin species.