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

We have isolated and cloned a gene, designated AMD1 (ascidian MyoD-rel ated factor 1), and its cDNAs that encode a member of the family of my ogenic basic helix-loop-helix (bHLH) factors from the ascidian Halocyn thia roretzi. The AMD1 gene consists of four exons and is transcribed into at least two distinct mRNAs, which differ in their 3' untranslate d region. The gene encodes a protein of 435 amino acids, which exhibit s 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 detectabl e in fertilized eggs or in very early embryos. They were first detecte d at the 64-cell stage, a few hours prior to the accumulation of mRNAs for embryonic muscle-specific proteins. In addition, expression of th e 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.