GENOMES UNCOUPLED1: an RNA-binding protein required for early PSII biogenesis

Abstract

AbstractGenomes Uncoupled1 (GUN1), a nuclear-encoded chloroplast pentatricopeptide repeat (PPR) protein, serves as a master integrator of diverse retrograde signals, mediating chloroplast-to-nucleus retrograde signaling. Although PPR proteins primarily function in organelle RNA metabolism, a target RNA of GUN1 remained unknown. This study reveals that GUN1 recognizespsbDtranscripts derived from a blue light-responsive promoter (BLRP), transcripts referred to aspsbDBLRP. Overexpression of GUN1 significantly reduces the levels ofpsbDBLRP, whereas the loss of GUN1 leads to the accumulation of the putative target RNA. Thein vitroRNA andin vivogenetic studies further demonstrate the critical role of the C-terminal small MutS-related (SMR) domain in stimulatingpsbDBLRP processing and PsbD (D2, a PSII core protein) synthesis. The GUN1-dependentpsbDBLRP processing promotes PSII biogenesis during the early seedling development and de-etiolation phase. This finding underscores the role of GUN1 as an RNA-binding protein, highlighting the essential function of the SMR domain in processing the target RNA.SignificanceBiogenic retrograde signaling is essential in regulating chloroplast biogenesis, and it entails a nuclear-encoded chloroplast protein, Genomes Uncoupled1 (GUN1). Since its initial discovery in 1993, thegun1mutant has been widely used to reveal the precise function of GUN1 in plastids as well as its downstream signaling components. Although GUN1 contains pentatricopeptide repeat motifs, it has been considered a non-canonical PPR protein because no one could demonstrate GUN1-RNA interaction in planta. However, through our computational analysis of PPR codes and subsequent biochemical studies, we were able to unravel its putative target RNA, namelypsbDBLRP. Therefore, our finding may lead to a reevaluation of GUN1 research in the context of RNA metabolism and a revision of our current understanding of GUN1.