Tissue-specific expression of the skeletal alpha-actin gene involves sequences that can function independently of MyoD and Id.

The skeletal alpha-actin gene is a member of the sarcomeric contractile protein gene family and is specifically expressed in differentiated muscle. The skeletal alpha-actin gene is regulated efficiently by enhancer and regulatory sequences between nucleotide positions -1282 and -87. In the present study we have shown that the sequences 3' of nucleotide position -87 can functionally interact with the SV40 enhancer in a tissue-specific manner and can restrict the ubiquitous function of the SV40 enhancer to myogenic cells. Site-specific cassette mutagenesis was used to delimit the sequences upstream of the TATA motif (-32), between nucleotide positions -64 and -37, that mediate efficient expression in myogenic cells in the presence of the SV40 enhancer. The skeletal alpha-actin promoter was trans-activated by the helix-loop-helix (HLH) transcription factors MyoD, MRF-4, and Myogenin, in pluripotential 10T1/2 fibroblasts and trans-repressed by the HLH protein Id (inhibitor of differentiation) in myogenic C2C12 cells. This trans-regulation required sequences upstream of -87 and occurred independently of the two consensus E boxes (CANNTG) at positions +18 and +71. The -64/-37 region interacted with purified Sp1 and an unidentified protein(s), proximal regulatory factor(s) I (PRF-I). We conclude that the muscle-specific expression of the skeletal alpha-actin promoter is not simply determined by MyoD elements and enhancer and regulatory sequences, but that the minimal promoter contains important determinants of cell-specific transcription that can function independently of the helix-loop-helix transcription factors.