Regulation of plastid gene transcription by sigma factor binding protein

Abstract

In chloroplasts, many photosynthesis-related genes are transcribed tissue-specifically and/or light responsively by E.coli-type RNA polymerase (PEP). Core subunits of PEP are encoded by the plastid genome, while the promoter recognition is mediated by sigma factors that are encoded by the nuclear genome. The differential usage of the sigma factors is considered to play the pivotal role for the tissue-specific and/or light responsive transcription. Here, we identified a novel protein termed SibI that specifically binds to the region 4 (-35 element recognition domain) of a sigma factor (At Sig1) by yeast two-hybrid screening. SibI had no sequence similarity to any proteins of known function, but the N-terminal region of SibI had a feature of the chloroplast transit peptide. Transient expression assay of SibI-GFP fusion protein in protoplasts showed that SibI was imported into chloroplasts. We established a new run-on assay system utilizing transformed protoplasts, and found that the transient expression of SibI caused the gene specific transcriptional activation with various degrees depending on the protoplast preparations. These results suggest that the nuclear-encoded sigma factor binding protein, SibI, is involved in the nuclear-control over the plastid gene transcription. The molecular mechanism of the SibI function in the plastid gene transcription will be discussed.