Identification of a Chloroplast-Localized SAP Domain Containing Protein in Arabidopsis thaliana

Abstract

Plastids are plant-specific organelles that originated from cyanobacteria, and possess their own genome and generic system. In higher plants, plastids may differentiate into several forms, such as chloroplasts and amyloplasts. Unlike cyanobacteria, plastid genes in higher plants are transcribed by two distinct RNA polymerases: plastid encoded eubacterial-type RNA Polymerase (PEP) and nuclear encoded phage-type RNA Polymerase (NEP). It has been established that plastid transcription plays an important role in plastid differentiation. However, molecular mechanism underlying plastid differentiation is still unclear. To identify genes involved in nuclear control of plastid differentiation, we screened for albino mutants in RIKEN phenome database and found 12 candidates. Among them, we focused on At3g04260, since it is predicted to be localized in chloroplasts and has been identified in several plastid transcription complex proteomes. At3g04260 contains a SAP domain that is involved in DNA binding in eukaryotic matrix attachment region binding proteins. Transient and Gel shift assays suggested that the N-terminal sequences of At3g04260 act as a plastid transit peptide and that its SAP domain had DNA binding activity. To understand the role of At3g04260 in plastid differentiation, we isolated T-DNA insertion mutant of At3g04260. As expected, the mutant showed an albino phenotype. The tilling microarray analysis showed that expression of all PEP-dependent genes including photosynthesis and rRNA genes were significantly reduced, however NEP-dependent transcripts were not decreased in the mutant plants. These results suggested that the plastid localized DNA binding protein, At3g04260 is involved in the regulation of plastid gene expression during chloroplast development.