THE SINGLE MYOD FAMILY GENE OF CIONA-INTESTINALIS ENCODES 2 DIFFERENTIALLY EXPRESSED PROTEINS - IMPLICATIONS FOR THE EVOLUTION OF CHORDATE MUSCLE GENE-REGULATION

A MyoD family gene was identified in the ascidian Ciona intestinalis a nd designated CiMDF (Ciona intestinalis Muscle Determination Factor). Expression of CiMDF was restricted to the muscle cells of the developi ng embryo and the body-wall muscle of adults. Northern blots showed th at two differentially regulated CiMDF transcripts were expressed durin g development. A 1.8 kb transcript (CiMDFa) appeared first and was gra dually replaced by a 2.7 kb transcript (CiMDFb). These transcripts enc oded essentially identical MyoD family proteins with the exception of a 68 amino acid C-terminal sequence present in CiMDFb that was absent from CiMDFa. Although both CiMDFa and CiMDFb contained the cysteine-ri ch/basic-helix loop helix domain (Cys-rich/bHLH) present in all MyoD f amily proteins, only CiMDFb contained the region near the C terminus ( Domain III) characteristic of this gene family. Genomic Southern blots showed that C. intestinalis has only one MyoD family gene, suggesting that CiMDFa and CiMDFb result from differential processing of primary transcripts. The existence of two MyoD family proteins that are diffe rentially expressed during ascidian embryogenesis has novel parallels to vertebrate muscle development and may reflect conserved myogenic re gulatory mechanisms among chordates.