Functional analysis of the promoter region of amphioxus β-actin gene: a useful tool for driving gene expression in vivo.

Amphioxus is a promising new animal model for developmental biology. To develop molecular tools for this model, we characterized the promoter region of a cytoplasmic β-actin gene (Bb-actin-6-2) from the Chinese amphioxus Branchiostoma belcheri. In situ hybridization and real time-quantitative PCR analyses showed that this gene is expressed in many tissues throughout embryonic development. Cloning of cDNA revealed two isoforms with distinct transcription start sites. Isoform #1 exhibits a similar exon/intron and regulatory element organization to that of vertebrate β-actin, whereas isoform #2 lacks the first exon of isoform #1 and recruits its first intron as a promoter. The activities of upstream promoter regions in the two isoforms were examined using the lacZ reporter system in amphioxus embryos. The proximal promoter of isoform #1 drove reporter gene expression broadly in 58.6 % of injected embryos. That of isoform #2 exhibited much higher activity (91.5 %) than that of isoform #1 or the human EF-1-α gene (38.2 %). We determined the minimal promoter regions of the two isoforms via functional analysis. These two regions, alone or inserted a random DNA fragment upstream, had no detectable activity, but when an upstream enhancer was inserted, the promoters directed reporter gene expression in 61.0 and 93.8 %, respectively, of injected embryos in a tissue-specific manner. Our study not only provides insight into the regulatory mechanism underlying amphioxus Bb-actin-6-2 gene expression, but also identifies two sets of efficient proximal and minimal promoters. These promoters could be used to construct gene expression vectors for transgenic studies using amphioxus as a model.