GUN1 (GENOMES UNCOUPLED1) Encodes a Pentatricopeptide Repeat (PPR) Protein Involved in Plastid Protein Synthesis-Responsive Retrograde Signaling to the Nucleus

Abstract

Plastid-to-nucleus signaling coordinates the expression of nuclear and plastid genes required for the assembly of functional chloroplasts. We have isolated new alleles of gun1 (genomes uncoupled1) by screening EMS-mutagenised lines of Arabidopsis thaliana containing the GFP reporter gene under the control of a tobacco RbcS promoter for GFP expression in the presence of norflurazon, a carotenoid biosynthesis inhibitor, or lincomycin, an inhibitor of plastid translation. gun1 mutants were able to express photosynthesis- related nuclear genes in the presence of lincomycin, unlike other gun (gun2-gun5) mutants. Microarray analysis identified CA1 (CARBONIC ANHYDRASE1) as the gene most responsive to lincomycin in wild-type seedlings, and a CA1 promoter::GFP reporter gene was used to map the gun1 mutation to a 224 kb region on chromosome 2. Sequencing of candidate genes identified GUN1 as At2g31400, encoding a pentatricopeptide repeat (PPR) protein of 918 amino acid residues. gun1-100 contained a point mutation creating a stop codon at amino acid 56, whereas gun1-1 contained a point mutation resulting in an Ala259Val change. GUN1-GFP fusion proteins were targeted to chloroplast nucleoids following microprojectile bombardment of Arabidopsis and tobacco leaves.