The structural complexities of the myelin basic protein gene from mouse are also present in shark

The Golli-mbp gene complex contains two overlapping transcription units with two distinct promoters, of which the downstream (myelin basic protein [mbp]) promoter is more frequently used. A previous comparison of the downstream promoter sequences from shark and mouse allowed the identification of two DNA sequences called the boxes I and II and the wobble zone. The boxes I and II sequence is a composite cis-acting motif that is thought to be involved in the regulation of the downstream promoter. It contains sequences similar to T-antigen, MyoD/E2A, and glucocorticoid receptor-binding sites. The wobble zone codes for an exon (5a in the nomenclature of Campagnoni et al., 1993) that is included in messenger RNAs transcribed from the upstream promoter. The polypeptides encoded by this exon from shark and mouse are 86 and 84 amino acids long, respectively. These polypeptides are overall 59% identical and include a region (residues 41-75 in shark and 39-73 in mouse) that is 89% identical between the two species. A primary sequence analysis showed that each of these polypeptides contains an N-glycosylation site, phosphorylation sites for Ca2+/calmodulin-dependent protein kinase, protein kinase C and casein kinase II, and partial ATP- and GTP-binding sites. The shark polypeptide also contains a phosphorylation site for proline-directed protein kinase. These observations are consistent with the notion that the intricate structure and regulation of the Golli-mbp gene complex arose during vertebrate evolution within a common ancestor to sharks and mammals.