Two Novel Proteins, MRL7 and Its Paralog MRL7-L, Have Essential but Functionally Distinct Roles in Chloroplast Development and Are Involved in Plastid Gene Expression Regulation in Arabidopsis

Chloroplast development requires the coordinated action of various proteins, many of which remain to be identified. Here, we report two novel genes, Mesophyll-cell RNAi Library line 7 (MRL7) and MRL7-Like (MRL7-L), that are involved in this process. An Arabidopsis knock-down transgenic plant (MRL7-RNAi) with delayed-greening phenotype was isolated from an RNA interference (RNAi) transformant library. Cotyledons and young leaves of MRL7-RNAi were pale in seedlings and gradually greened as the plant matured, while a knock-out in the MRL7 gene was seedling lethal. The MRL7 protein was shown to co-localize with a marker protein for nucleoids in chloroplasts, indicative of a role for the protein in chloroplast nucleic acid metabolism. Accordingly, chloroplast development was arrested upon loss of MRL7 function and the expression of plastid-encoded genes transcribed by plastid-encoded RNA polymerase (PEP) was significantly reduced in MRL7 knock-down and knock-out plants. A paralog of MRL7 (MRL7-L) was identified in the Arabidopsis genome. Both MRL7 and MRL7-L are only found in land plants and encode previously uncharacterized proteins without any known conserved domain. Like MRL7, knock-down of MRL7-L also resulted in a virescent phenotype, and a similar effect on plastid gene expression. However, the MRL7-L protein was localized to the chloroplast stroma. Taken together, our data indicate that the two paralogous proteins MRL7 and MRL7-L have essential but distinct roles during early chloroplast development and are involved in regulation of plastid gene expression.