N-terminus plus linker domain of Mg-chelatase D subunit is essential for Mg-chelatase activity in Oryza sativa

Mg chelatase, a key enzyme in chlorophyll biosynthesis, is comprised of I, D and H subunits. Among these subunits, the D subunit was regarded to mediate protein interactions due to its unique protein domains. However, the functional roles of the different domains of the D subunit in vivo remain unclear. In this study, we dissected the rice (Oryza sativa) D subunit (OsCHLD) into three peptide fragments: the putative chloroplast transit peptide (TP, Met1 to Arg45), the N-terminus plus linker domain (OsCHLDN + L, Ala46 to Leu485) and the C-terminus (OsCHLDC, Ile486 to Ser754), to explore the roles of these fragments. The results of the yeast two-hybrid assay and the in vitro reconstitution of the Mg-chelatase activity showed that only OsCHLDN + L interacted with the I and H subunits and maintained most of the Mg-chelatase activity in vitro. Furthermore, artificial TP-OsCHLDN + L and TP-OsCHLDC were overexpressed in rice. Interestingly, an incomplete co-suppression had occurred in both of the overexpressed (OsCHLDN + L-ox and OsCHLDC-ox) plants, resulting in a significantly downregulated expression of endogenous OsCHLD. Therefore, these transgenic plants had adequate OsCHLDN + L and OsCHLDC instead of endogenous OsCHLD, providing ideal models to study the function of different domains of the D subunit in vivo. The OsCHLDN + L-ox plants showed an identical phenotype to that of the wild type, while the OsCHLDC-ox plants demonstrated a yellowish phenotype that resembled the D subunit mutants. These results indicated that only OsCHLDN + L could complement the function of endogenous OsCHLD, providing direct evidence that OsCHLDN + L is essential for Mg-chelatase activity in vivo.