Genome-wide identification and analysis of the PLP_deC genes involved in taurine synthesis in bivalves

Abstract

Bivalves, such as oysters and scallops, are rich in taurine and play crucial roles in marine ecosystems as well as in aquaculture. However, mechanisms governing the taurine biosynthesis in bivalves remain poorly understood. Cysteine sulfinic acid decarboxylase (CSAD) and glutamate decarboxylase-like 1 (GADL1), as members of pyridoxal phosphate-dependent decarboxylase (PLP_deC) family, catalyze the decarboxylation step in taurine synthesis. To investigate their evolution and function in bivalves, a genome-wide identification of these genes was conducted in five bivalve species. A total of 61 PLP_deC genes were identified, of which 23 were predicted to be involved in taurine synthesis. Phylogenetic analysis revealed that these genes cluster into two distinct groups: CSAD/GADL1-like and GAD-like. Notably, bivalves possess only one Gad gene, in contrast to the two typically found in vertebrates, whereas they harbor three to five Csad/Gadl1 genes, compared with just two in vertebrates. Three bivalve Csad genes exhibited high expression levels across most of developmental stages and adult tissues, suggesting their essential roles in development and the maintenance of normal physiological activities. Molecular docking analysis revealed that oyster CSADs exhibit higher substrate-binding specificity for cysteine sulfinic acid, while GAD shows higher specificity for glutamate. Heterologous overexpression assays demonstrated that oyster CSAD and GAD can significantly increase cellular taurine levels. This study is the first to provide evidence of the expansion of Csad genes in bivalve genomes and their involvement in taurine synthesis, offering novel insights into understanding the molecular mechanisms underlying the high taurine content in bivalves.