EXPRESSION OF AMD1, A GENE FOR A MYOD1-RELATED FACTOR IN THE ASCIDIAN HALOCYNTHIA-RORETZI

We have isolated and cloned a gene, designated AMD1 (ascidian MyoD-related factor 1), and its cDNAs that encode a member of the family of myogenic basic helix-loop-helix (bHLH) factors from the ascidian Halocynthia roretzi. The AMD1 gene consists of four exons and is transcribed into at least two distinct mRNAs, which differ in their 3' untranslated region. The gene encodes a protein of 435 amino acids, which exhibits homology to the bHLH domain of other myogenic bHLH factors including vertebrate MyoD1. A reverse transcription-polymerase chain reaction (RT-PCR) assay revealed that transcripts of the gene were not detectable in fertilized eggs or in very early embryos. They were first detected at the 64-cell stage, a few hours prior to the accumulation of mRNAs for embryonic muscle-specific proteins. In addition, expression of the AMD1 gene was evident in adult body-wall muscle but not in heart and other non-muscle tissues. These results suggest the possibility that, in ascidians as in vertebrates, the myogenic bHLH factor is involved in the specification of embryonic cells as myogenic cells.