Characterization of Plastid-Encoded RNA Polymerase-Associated Protein 3 in Development of Chloroplast in Arabidopsis

Abstract

Plastid-encoded RNA polymerase (PEP) is essential for chloroplast development by controlling expression of plastid genes. Previous studies revealed that PEP activity is determined by the successful assembly of PEP complex which is composed of plastid-encoded core proteins and its associated proteins (PEP-associated proteins, PAPs) encoded in the nucleus. It has recently been reported that the extensive interactions between proteins composing a PEP complex, such as PEP-PAP or PAP-PAP, are pivotal to establish a functional PEP complex. In this study, an Arabidopsis mutant showing whitening and seedling-lethal phenotype was isolated. Genetic and molecular characterization showed that the mutant contains a lesion in pTAC10/PAP3 gene that encodes an S1 RNA binding domain-containing protein. Expression analyses of ptac10-1 mutants and pTAC10-overexpressing transgenic plants showed that pTAC10 expression promotes chloroplast development. Interaction analyses between pTAC10 and PEP complex subunits displayed that pTAC10 interacts with PAP proteins including FSD2, FSD3, pTAC7, pTAC14, TrxZ and pTAC10 itself but not with PEP core proteins like rpoA or rpoB. Moreover, interaction analyses using variously truncated pTAC10s showed that the S1 domain is not involved in the pTAC10-PAP interactions although it is essential for pTAC10 function. Rather, C-terminal region downstream of the S1 domain is responsible for the pTAC10-PAP interactions. Furthermore, introducing DNA constructs overexpressing a series of C-terminal truncated pTAC10s could not recover the ptac10-1 phenotype and induced an abnormal whitening phenotype in the wild type, suggesting that chloroplast development would be disrupted by interrupting the assembly of PEP complex. These observations suggested that pTAC10 plays a decisive role in establishing the functional PEP complex by interacting with other PAPs, which is required for chloroplast development.