Siphon-Specific Expression of an Actin Encoding Gene Is Regulated by Six1/2 in Ciona savignyi

Actin is a ubiquitous protein and plays essential roles on cellular structure maintenance and cellular motility in both muscle and non-muscle tissues. Multiple genes encoding muscle actin have been identified from the ascidians, including those expressed in the larval tail muscle, the adult body-wall muscle, and adult heart muscle. In this study, a novel striated non-tail muscle actin gene was identified from the RNA-seq data of Ciona savignyi embryos. Phylogenetic analysis, alignment of the N-terminal amino acid sequences and comparation of diagnostic residues provided evidence that it had high similarity with vertebrate cardiac and skeletal muscle actin. In situ hybridization and promoter-driven GFP reporter assay revealed that it was specifically expressed in the primordia of the oral and atrial siphon. We hereby defined it as siphon-specific muscle actin coding gene ( Cs-SMA ). A 201 bp (−1350 bp to −1150 bp) sequence containing T-box and Six1/2 binding motif within the upstream region of Cs-SMA confined the expression of GFP in the siphons of electroporated embryos. Six1/2 binding motif was experimentally confirmed to play indispensable role in controlling the siphon-specific expression of Cs-SMA. The tissue-specific expression of Cs-SMA in the siphon primordia indicated its potential crucial roles in Ciona embryogenesis and organogenesis.