Genomic organization and the 5'-upstream sequence of the rat cytosolic sialidase gene

We have previously cloned a cDNA of a rat cytosolic sialidase which is strongly expressed in skeletal muscle. Both the highest enzyme activity, as well as the highest mRNA level, are present in this tissue. To understand the basis of the expression of this sialidase, we have cloned and sequenced the rat gene and its 5'-upstream region from a rat genomic library. The gene encoding the 1.8 kb skeletal muscle mRNA was found to span 3.4 kb of genomic DNA, and to consist of two introns and three exons. Exon 1 contains the 5' noncoding region, and exons 2 and 3 encode the regions containing the AUG initiation codon and two Asp-boxes, respectively. In the 5'-upstream sequence, there are a TATA box and two E-box pairs known as consensus binding sites for muscle-specific transcription factors. Analysis of the expression of transfected sialidase enhancer/promoter expression plasmid demonstrated the sialidase enhancer/promoter to be active in rat L6 myogenic cells shown to express this gene, but inactive in rat 3Y1 fibroblasts shown not to express the enzyme. The transcription activity was increased 3-fold after induction of myoblast differentiation by serum depletion. These observations give an account of constitutive expression of the sialidase gene in skeletal muscle.