Analysis of the structure and expression of mammalian myoglobin genes.

The myoglobin gene is a distant member of the globin gene superfamily, expressed in muscle rather than erythroid cells. In vertebrate muscle, myoglobin is the principle haemoprotein and serves to facilitate the diffusion of oxygen from the vascular system to the muscle mitochondria. A single gene encodes myoglobin of both cardiac and skeletal muscle in the mouse. The characterisation of the mouse myoglobin gene established that it has the same three exon/two intron structure found in the ?- and ?-globin genes but differs from haemoglobin genes in having very long non-coding DNA regions. These features are shared by human and seal myoglobin genes. Sequence comparison of human, seal and mouse myoglobin genes revealed a highly conserved upstream domain, approximately 120 bp before the myoglobin gene CAP site and extending over 200 bp. Although the position before the CAP site suggests it may play some regulatory role, this sequence has no homology to conserved 5'-flanking sequences described for contractile protein genes. As expected from the early appearance of low levels of myoglobin mRNA in embryonic skeletal muscle, myoglobin gene expression is induced in both rat and mouse embryonic myoblasts during differentiation in vitro, concomitant with the contractile protein genes. Myoglobin-CAT gene fusions containing -1 kb of 5'-myoglobin DNA, including the conserved domain, are expressed on introduction into mouse myoblasts and furthermore, expression increases during myoblast differentiation over a transient expression period. This should provide a good in vitro system to study myoglobin gene expression at the molecular level during myogenesis.