Structure of Ubiquitin Genes in Artemia
Ubiquitin is a small protein (76 aa) present in all eukaryotic organisms studied, that has been involved in several processes, all of which imply the covalent attachment of ubiquitin by its C-terminal glycine to the free amino groups of other proteins. Ubiquitin genes are unique in that they are organized as intronless tandem repetitions of the coding unit (228 bp). These genes are translated as polyproteins then cleaved specifically to ubiquitin units.
We have isolated and sequenced a truncated cDNA clone that contains three units (one of them incomplete) of ubiquitin plus 240 bp of 3’ nontranslated sequence. Using this cDNA as a probe, we have isolated three different genomic clones in lambda. One of these phages, λgU-5 has been studied in detail. It contains five ubiquitin units. The 5’ upstream region has consensus promoter signals, as the TATA box (position -27) and the CAAT box (position -117). The first ATG is at position 53 after the putative transcription initiation site. In both the cDNA and the gene there is an extension of 13 (12) amino acid codons, very similar but not identical, between the last ubiquitin unit and the stop codons.
Analysis of mRNAs shows two bands of 2.7 and 3.5 kb. This finding suggests that the gene found in λgU-5 is not the major transcription unit except for the presence of long 3’ non-translated regions.