The isolation and functional identification of a phosphoenolpyruvate carboxykinase gene from Saccharina japonica

Similar to other macroalgae, Saccharina japonica has CO2-concentrating mechanism (CCM) that allows high photosynthesis efficiency and elevates biomass. Phosphoenolpyruvate carboxykinase (PEPCK) in cytoplasm is an essential component of CCM. However, no reports on cytosolic PEPCK of S. japonica are available. In this study, the full-length cDNA sequence of a PEPCK gene (SjPCK1) predicted to be localized in cytoplasm, was screened from the full-length transcriptome of S. japonica gametophytes and identified by RT-PCR. The complete cDNA sequence of SjPCK1 was 2174 bp in length, which encompassed an open reading frame (ORF) of 1734 bp, a 5′- untranslated region (UTR) of 216 bp and a 3′-UTR of 224 bp. In parallel, the genomic DNA of SjPCK1 was 21 294 bp in length, characterized by the presence of 11 introns and 12 exons. It encoded a protein of 577 amino acids with a molecular weight of 63 kD and an isoelectric point of 8.47. Amino acid sequence alignment showed that the functional sites of PEPCK were highly conserved in the selected species. Phylogenetic analysis and sequence characterization showed that SjPCK1 was an ATP-dependent PEPCK. SjPCK1 gene was expressed by using Escherichia coli prokaryotic expression system, and the SjPCK1 protein with His 6 tag (rSjPCK1) was 81.93 kD in molecular weight. Enzyme activity assay results showed that the specific activity of carboxylase and decarboxylase of rSjPCK1 was 1.91 ± 0.29 U/mg prog and 11.3 ± 1.97 U/mg prog, respectively. These findings provide valuable molecular and biochemical insights for a further analysis of the role of cytosolic PEPCK in the storage of inorganic carbon in S. japonica.