Identification of a cis acting element responsible for muscle specific expression of the c-mos protooncogene

A series of deletion constructs of the 5' flanking region of rat c-mos gene was positioned upstream to the CAT gene and transfected into muscle and non-muscle cells. CAT activities revealed that a region located downstream of a TATA box and containing the proximal transcription start site is the muscle c-mos promoter. This promoter is more efficient in L6 alpha 1 myoblasts than in L6 alpha 1 myotubes but not in C3H10T1/2 cells. Gel shift assays demonstrated that nuclear proteins from myoblasts and myotubes formed complexes migrating differently. Footprinting analyses showed that nuclear proteins from L6 alpha 1 myoblasts protected a DNA fragment located at position nt -979 to nt -938 relative to the first ATG of the rat c-mos ORF while nuclear proteins from myotubes protected the DNA between nt -998 to nt -928. Furthermore one of protein - DNA complexes containing the proximal transcription start site, included a consensus sequence TGTC(AGT/TCG)CC(A/T)G present in the initiator element (Inr) of several genes. Southwestern blot analysis pointed to a 82kDa polypeptide as a potential candidate for trans acting factor in myoblasts. In L6 alpha 1 myotubes this polypeptide is replaced by other proteins of 40-42kDa and 82kDa. An interplay between these two complexes may constitute a developmental as well as a physiologically regulated mechanism that modulates c-mos expression during the early stages of myogenesis.