REGULATION OF THE RAT MUSCLE GLYCOGEN PHOSPHORYLASE-ENCODING GENE DURING MUSCLE-CELL DEVELOPMENT

The muscle isozyme of glycogen phosphorylase (MGP) catalyzes the hydro lysis of intracellular glycogen in mammalian tissues and is produced i n skeletal muscle, brain and heart. The MGP gene is developmentally an d neurally regulated in skeletal muscle, but little is known about the gene's transcriptional regulation. We have isolated and characterized the 5' flanking region of rat MGP. Truncated portions of the MGP 5' f lanking region were coupled to the bacterial cat reporter gene and use d in transient transfection assays in the mouse muscle C2C12 cell line . The region between -211 and +62 contained the smallest regulatory do main capable of demonstrating developmentally regulated myogenic expre ssion in C2C12 cells. This was in contrast with findings from another investigation that transfected this cell line with human MGP [Lockyer and McCracken, J. Biol. Chem. 266 (1991) 20262-20269]. A 172-nucleotid e (nt) region between -839 and -666 functioned as a potent enhancer in C2C12 cells when coupled to its cognate promoter, but not when couple d to a simian virus 40 promoter. This rat MGP enhancer region is 78% i dentical to a comparable region of the human MGP 5' flanking region, b ut contains only one putative regulatory element that has been previou sly identified in other muscle genes. These data suggest that rat MGP transcription in C2C12 muscle cells is modulated by a potent enhancer that utilizes novel regulatory elements.