Sigma factor 1 in chloroplast gene transcription and photosynthetic light acclimation.

Lauren A Macadlo,Iskander M Ibrahim,Sujith Puthiyaveetil,Robert Sharwood

doi:10.1093/jxb/erz464

Lauren A Macadlo, Iskander M Ibrahim + Show 2 more

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https://doi.org/10.1093/jxb/erz464

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Abstract

Sigma factors are dissociable subunits of bacterial RNA polymerase that ensure efficient transcription initiation from gene promoters. Owing to their prokaryotic origin, chloroplasts possess a typical bacterial RNA polymerase together with its sigma factor subunit. The higher plant Arabidopsis thaliana contain as many as six sigma factors for the hundred or so of its chloroplast genes. The role of this relatively large number of transcription initiation factors for the miniature chloroplast genome, however, is not fully understood. Using two Arabidopsis T-DNA insertion mutants, we show that sigma factor 1 (SIG1) initiates transcription of a specific subset of chloroplast genes. We further show that the photosynthetic control of PSI reaction center gene transcription requires complementary regulation of the nuclear SIG1 gene at the transcriptional level. This SIG1 gene regulation is dependent on both a plastid redox signal and a light signal transduced by the phytochrome photoreceptor.

Highlights

Sigma factors are subunits of bacterial RNA polymerases.They enable efficient transcription of bacterial genes by their three distinct activities: by imparting a promoter recognition property to the RNA polymerase; by melting the double-stranded promoter regions into transcription-competent, singlestranded open complexes; and by interacting with other DNAbinding transcription factors for regulated gene expression (Paget and Helmann, 2003; Davis et al, 2017)
Using two Arabidopsis T-DNA insertion mutants, we show that sigma factor 1 (SIG1) initiates transcription of a specific subset of chloroplast genes
The SIG1 transcript reduction is, more marked in sig1-2. sig1-1 is a confirmed homozygous SALK line with a T-DNA insertion in the 3'-untranslated region (UTR) of the SIG1 gene, while sig1-2 is a confirmed homozygous GABI-Kat line that carries an insertion within the penultimate exon (Supplementary Figs S1, S2)

Summary

Introduction

Sigma factors are subunits of bacterial RNA polymerases.They enable efficient transcription of bacterial genes by their three distinct activities: by imparting a promoter recognition property to the RNA polymerase; by melting the double-stranded promoter regions into transcription-competent, singlestranded open complexes; and by interacting with other DNAbinding transcription factors for regulated gene expression (Paget and Helmann, 2003; Davis et al, 2017). Chloroplasts are cytoplasmic organelles in which photosynthesis takes place in plants and algae By virtue of their cyanobacterial ancestry, chloroplasts contain a small transcriptionally active genome and a bacterial gene expression machinery (Keeling, 2010). In flowering plants and moss, one or more single-subunit phage-type RNA polymerases, known as the nuclear-encoded polymerases (NEPs), transcribe a small subset of chloroplast genes from distinct promoter elements (Börner et al, 2015). These NEP-transcribed genes include rpoB, encoding the β-subunit of PEP, and a few tRNA genes.

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