Molecular cloning and sequence analysis and the response of a aryl hydrocarbon receptor homologue gene in the clam Ruditapes philippinarum exposed to benzo(a)pyrene

The aryl hydrocarbon receptor (AhR) is a member of the basic helix–loop–helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. In the present study, a cDNA encoding an AhR homologue was initially cloned and sequenced from the clam, Ruditapes philippinarum. The predicted amino acid sequences contain regions characteristic of AhRs from other species including basic helix–loop–helix (bHLH) and Per-ARNT-Sim (PAS) domains, but it does not contain distinct Q-rich domain as found in the mammalian AhRs. The clam AhR homologue cDNA coded for a 765 amino acids protein and phylogenetic analysis demonstrated that it was clustered within the invertebrate AhRs branch. The clam AhR homologue mRNA expression was detected in all the adult tissues tested (gill, digestive gland, adductor muscle and mantle) and highest transcription level were observed in gill compared to other tissues.Quantitative real-time RT-PCR analysis revealed that the clam AhR homologue mRNA expression levels in gill and digestive gland of R. philippinarum were induced by benzo(a)pyrene (BaP) and the absolute expression levels of the gene showed temporal and dose-dependent response.