The CArG promoter sequence is necessary for muscle-specific transcription of the cardiac actin gene in Xenopus embryos.

Promoter sequences required for activation of the Xenopus cardiac actin gene in embryonic muscle were analysed by micro-injecting chimeric actin/beta-globin genes into the two-cell Xenopus embryo. Transcription was monitored during subsequent differentiation of embryonic muscle and non-muscle tissues. The effect of a variety of mutations including internal deletions and linker scan mutations between -64 and -396 within the cardiac actin promoter were tested. This region contains four copies of a conserved motif, the CArG box, common to vertebrate striated muscle acting gene promoters. In the Xenopus cardiac actin gene, the most proximal of these motifs (CArG box 1) located at -80, was essential for muscle-specific transcription. Other CArG motifs could functionally substitute for CArG box 1 when placed in this position. CArG boxes 3 and 4 bound the same activity in a neurula embryo nuclear extract as CArG box 1 and the amount of this binding activity was constant through early development.