An isoform of the plastid RNA polymerase-associated protein FSD3 negatively regulates chloroplast development

Sangyool Lee,Young Hee Joung,Yang Do Choi,Ju-Kon Kim,Geupil Jang

doi:10.1186/s12870-019-2128-9

Abstract

BackgroundPlastid-encoded RNA polymerase (PEP) plays an essential role in chloroplast development by governing the expression of genes involved in photosynthesis. At least 12 PEP-associated proteins (PAPs), including FSD3/PAP4, regulate PEP activity and chloroplast development by modulating formation of the PEP complex.ResultsIn this study, we identified FSD3S, a splicing variant of FSD3; the FSD3 and FSD3S transcripts encode proteins with identical N-termini, but different C-termini. Characterization of FSD3 and FSD3S proteins showed that the C-terminal region of FSD3S contains a transmembrane domain, which promotes FSD3S localization to the chloroplast membrane but not to nucleoids, in contrast to FSD3, which localizes to the chloroplast nucleoid. We also found that overexpression of FSD3S negatively affects photosynthetic activity and chloroplast development by reducing expression of genes involved in photosynthesis. In addition, FSD3S failed to complement the chloroplast developmental defects in the fsd3 mutant.ConclusionThese results suggest FSD3 and FSD3S, with their distinct localization patterns, have different functions in chloroplast development, and FSD3S negatively regulates expression of PEP-dependent chloroplast genes, and development of chloroplasts.

Highlights

Plastid-encoded RNA polymerase (PEP) plays an essential role in chloroplast development by governing the expression of genes involved in photosynthesis
PEP forms a complex with PEP-associated proteins (PAPs), and the Arabidopsis thaliana nuclear genome contains at least 12 Plastid-encoded RNA polymeraseassociated protein (PAP) genes [3, 12], and all PAPs have been identified in the nucleoid or transcriptionally active chromosome (TAC) proteomes [13,14,15,16]
All 12 Arabidopsis PAP genes contain introns, suggesting that variant PAPs might be involved in the formation of the PEP complex and chloroplast development. pTAC12/PAP5 encodes two different protein isoforms in maize and both variant proteins assemble into the PEP complex [27], the pTAC12/PAP5 isoforms are not produced by alternative splicing, but by post-transcriptional processes such as alternative initiation of translation or differential proteolytic cleavage

Summary

Introduction

Plastid-encoded RNA polymerase (PEP) plays an essential role in chloroplast development by governing the expression of genes involved in photosynthesis. NEP is a single-subunit RNA polymerase and PEP is a multimeric RNA polymerase composed of four core proteins, rpoA, rpoB, rpoC1, and rpoC2 [1,2,3]. Both NEP and PEP are required for chloroplast development [4, 5]. Previous genetic approaches have demonstrated the essential role of PAPs in the regulation of PEP activity and chloroplast development.

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